FN/Mats Henricson and Erik Nyquist
Copyright © 1990-1992 by
Ath: þessari HTML útgáfu af R&R var hnuplað frá Plús Plús hf.
Þeir eiga allan heiðurinn af henni!
(Sorry, no translation for the sentence about the conversion to HTML.)
Permission is granted to any individual or institution to use, copy, modify, and distribute this document, provided that this complete copyright and permission notice is maintained intact in all copies.
Ellemtel Telecommunication Systems Laboratories makes no representations about the suitability of this document or the examples described herein for any purpose. It is provided "as is" without any expressed or implied warranty.
Original translation from Swedish by Joseph Supanich in 1992
const
Member FunctionsThe purpose of this document is to define one style of programming in C++. The rules and recommendations presented here are not final, but should serve as a basis for continued work with C++. This collection of rules should be seen as a dynamic document; suggestions for improvements are encouraged. A form for requesting new rules or changes to rules has been included as an appendix to this document. Suggestions can also be made via e-mail to one of the following addresses:
Programs that are developed according to these rules and recommendations should be:
In order to reach these goals, the programs should:
Questions of design, such as how to design a class or a class hierarchy, are beyond the scope of this document. Recommended books on these subjects are indicated in the chapter entitled References.
In order to obtain insight into how to effectively deal with the most difficult aspects of C++, the examples of code which are provided should be carefully studied. C++ is a difficult language in which there may be a very fine line between a feature and a bug. This places a large responsibility upon the programmer. In the same way as for C, C++ allows a programmer to write compact and, in some sense, unreadable code.
Code written in bold type is meant to serve as a warning. The
examples often include class definitions having the format class
name {};
. These are included so that the examples
may be compiled; it is not recommended that class definitions be
written in this way. In order to make the code more compact, the
examples provided do not always follow the rules. In such cases, the
rule which is broken is indicated.
Many different C++ implementations are in use today. Most are based on the C++ Language System by AT&T. The component of this product which translates C++ code to C is called Cfront. The different versions of Cfront (2.0, 2.1 & 3.0 are currently in use) are referred to in order to point out the differences between different implementations.
Every time a rule is broken, this must be clearly documented.
enum
.typedef
declaration.&
) operator is used immediately after
the data type to indicate that the declared variable, constant, or
function argument is a reference.#define
statement. When this name appears in source
code, the compiler replaces it with the defined text string.virtual
.int
.class
, struct
,
union
, enum
, or typedef
definition or as an instantiation of a class template.catch
.Optimize code only if you know that you have a performance problem. Think twice before you begin.
If you use a C++ compiler that is based on Cfront, always compile
with the +w
flag set to eliminate as many warnings as
possible.
Various tests are said to have demonstrated that programmers
generally spend a lot of time optimizing code that is never executed.
If your program is too slow, use gprof++
or an equivalent
tool to determine the exact nature of the problem before beginning to
optimize.
Code that is accepted by a compiler is not always correct (in
accordance with the definition of the C++ language). Two reasons for
this are that changes are made in the language and that compilers may
contain bugs. In the short term, very little can be done about the
latter. In order to reduce the amount of code that must be rewritten
for each new compiler release, it is common to let the compiler
provide warnings instead of reporting errors for such code until the
next major release. Cfront provides the +w
flag to
direct the compiler to give warnings for these types of language
changes.
Include files in C++ always have the file name extension ".hh".
Implementation files in C++ always have the file name extension ".cc".
Inline definition files always have the file name extension ".icc".
An include file should not contain more than one class definition.
Divide up the definitions of member functions or functions into as many files as possible.
Place machine-dependent code in a special file so that it may be easily located when porting code from one machine to another.
The purpose of these conventions is to provide a uniform interpretation of file names. One reason for this is that it is easier to make tools which base their behaviour on the file name extension.
There are two kinds of include files in C++: those which contain code that is accepted by both ANSI-C and C++ compilers and those which contain code that is only accepted by C++ compilers. It is appropriate to distinguish between the two in order to avoid unpleasant compilation errors (from using the wrong kind of include file).
If a ".cc" file contains a large number of function definitions,
the object file produced by the compiler may be unnecessarily large.
In order to obtain the smallest possible executable files, it is
necessary to have a separate file for each function definition. This
is because the standard UNIX linker ld
links all
functions in an object file even if only one of them is actually used.
It is especially important to remember that virtual functions are
always linked. On the other hand, there are problems in managing a
large number of files, since sufficiently powerful tools are not
currently available. Also, the time necessary to compile a program
consisting of a large number of files is longer.
Some debuggers cannot debug inline functions. By placing inline
functions in a separate file and by including that file in the
implementation file, thus treating the inline functions as ordinary
functions, it is possible to debug the functions while testing the
program. For this to work some special preprocessor techniques must
be used. The inline definition file must not be included by the
include file for the class and the keyword inline
must be
removed.
When tools for managing C++ code are not available, it is much easier for those who use and maintain classes if there is only one class definition in each file and if implementations of member functions in different classes are not present in the same file.
Include files which contain code that is accepted by both C and C++ compilers should have the file name extension ".h".
When using a compiler that does not accept the extension ".cc", the extension ".C" is used instead.
No exceptions.
Inline definitions in a separate file for conditional compilation
// AnyClass.hh #ifndef OUTLINE #include "AnyClass.icc" #endif //AnyClass.cc #ifdef OUTLINE #define inline #include "AnyClass.icc" #undef inline #endif
Always give a file a name that is unique in as large a context as possible.
An include file for a class should have a file name of the form <class name> + extension. Use uppercase and lowercase letters in the same way as in the source code.
There is always a risk for name collisions when the file name is part of identifier names that are generated by the compiler. This is a problem in using any Cfront-based compiler.
AT&T's Cfront-based compiler creates two functions for every file in order to call constructors and destructors of static objects in the proper order. These functions are named:
char __sti__file_cc___Fv(); //filename is file.cc
char __std__file_cc___Fv(); //filename is file.cc
It is easily understood that if a program has two files with the same name but in different subdirectories, there will be name collisions between the functions generated above.
Since class names must generally be unique within a large context, it is appropriate to utilize this characteristic when naming its include file. This convention makes it easy to locate a class definition using a file-based tool.
Every file that contains source code must be documented with an introductory comment that provides information on the file name and its contents.
All files must include copyright information.
All comments are to be written in English.
Write some descriptive comments before every function.
Use //
for comments.
It is necessary to document source code. This should be compact and easy to find. By properly choosing names for variables, functions and classes and by properly structuring the code, there is less need for comments within the code.
Note that comments in include files are meant for the users of classes, while comments in implementation files are meant for those who maintain the classes.
All our code must be copyright marked. If the code has been developed over a period of years, each year must be stated.
The standardization of comments makes it possible to automatically generate man-pages from source code. This may be used to keep source code and documentation together until adequate tools for information management are available.
Comments are often said to be either strategic or tactical. A strategic comment describes what a function or section of code is intended to do, and is placed before this code. A tactical comment describes what a single line of code is intended to do, and is placed, if possible, at the end of this line. Unfortunately, too many tactical comments can make code unreadable. For this reason, it is recommended to primarily use strategic comments, unless trying to explain very complicated code.
If the characters //
are consistently used for writing
comments, then the combination /* */
may be used to make
comments out of entire sections of code during the development and
debugging phases. C++, however, does not allow comments to be nested
using /* */
.
No exceptions.
No exceptions.
No exceptions.
Documentation of a file
// // File: test.cc // Description: This is a test program // Rev: A // Created: Thur. Oct 31, 1991, 12:30:14 // Author: Erik Nyquist // mail<EMAIL_ADDRESS>// // Copyright Ellemtel Utvecklings AB 1991 // BOX 1505 // 125 25 ALVSJO // SWEDEN // tel int + 46 8 727 3000 // // The copyright to the computer program(s) herein // is the property of Ellemtel Utvecklings AB, Sweden. // The program(s) may be used and/or copied only with // the written permission of Ellemtel Utvecklings AB // or in accordance with the terms and conditions // stipulated in the agreement/contract under which // the program(s) have been supplied. //
Strategic and Tactical Comments
// THE NEXT TWO LINES ARE STRATEGIC COMMENTS // This function does some complicated things. It works like this: // blah-blah-blah ... int insanelyGreatAndComplicatedFunction( int i ) { int index = i++ + ++i * i-- - --i; // THIS IS A TACTICAL COMMENT return index; }
Every include file must contain a mechanism that prevents multiple inclusions of the file.
When the following kinds of definitions are used (in implementation files or in other include files), they must be included as separate include files:
Definitions of classes that are only accessed via pointers
(*
) or references (&
) shall not be
included as include files.
Never specify relative UNIX names in #include
directives.
Every implementation file is to include the relevant files that contain:
Use the directive #include
"filename.hh"
for user-prepared include files.
Use the directive #include
<filename.hh>
for include files from
libraries.
Every implementation file should declare a local constant string
that describes the file so the UNIX command what
can be
used to obtain information on the file revision.
Never include other files in an ".icc" file.
The easiest way to avoid multiple includes of files is by using an
#ifndef
/#define
block in the beginning of
the file and an #endif
at the end of the file.
The number of files included should be minimized. If a file is included in an include file, then every implementation file that includes the second include file must be re-compiled whenever the first file is modified. A simple modification in one include file can make it necessary to re-compile a large number of files.
When only referring to pointers or references to types defined in a
file, it is often not necessary to include that file. It may suffice
to use a forward declaration to inform the compiler that the class
exists. Another alternative is to precede each declaration of a
pointer to the class with the keyword class
.
True portable code is independent of the underlying operating
system. For this reason, relative UNIX search paths should be avoided
when including files. The processing of such search paths depends on
the compiler and UNIX should not be taken for granted. Instead,
search paths should be provided in make
files as options
for the compiler.
If a file only contains information that is only needed in an implementation file, that file should not be included in another include file. Otherwise, when the information is no longer needed in the implementation file, it may be necessary to re-compile each file that uses the interface defined in the include file.
Every C++ course teaches the difference between the include
directives for user-prepared and for library include files. If the
file name is bracketed between <
and
>
, the preprocessor will not search for the file in
the default directory. This reduces the risk of unintended name
collisions between user-prepared and library include files.
By declaring a local constant string, the compiler becomes
self-identifying. This may be used to easily determine the version of
the program that is used. The string must begin with the characters
@(#)
to be read by the UNIX what
command.
No exceptions.
No exceptions.
No exceptions.
No exceptions.
No exceptions.
Technique for preventing multiple inclusion of an include file
#ifndef FOO_HH #define FOO_HH // The rest of the file #endif
Never use explicit UNIX path names
// NOT RECOMMENDED #include <../include/fnutt.h> // NOT GUARANTEED TO WORK #include <sys/socket.h>
Local constant string for identifying implementation files.
static const char* sccsid = "@(#) Exception.cc, rev. A, Copyright Ellemtel Utvecklings AB 1991";
Include file for the class PackableString
// file: PackableString.hh #ifndef PACKABLESTRING_HH #define PACKABLESTRING_HH #include "String.hh" #include "Packable.hh" // It is not necessary to extern-declare class Buffer when // each pointer declaration specifies the keyword class as shown below. // An explicit extern-declaration makes the code easier to // understand. extern class Buffer; class PackableString : public String, public Packable { public: PackableString( const String& s ); class Buffer* put( class Buffer* outbuffer ); // ... }; #endif
Implementation file for the class PackableString
// PackableString.cc #include "PackableString.hh" // To be able to use Buffer-instances, Buffer.hh MUST be included. #include "Buffer.hh" Buffer* PackableString::put( Buffer* outbuffer ) { // ... }
The identifier of every globally visible class, enumeration type, type definition, function, constant, and variable in a class library is to begin with a prefix that is unique for the library.
The names of variables, constants, and functions are to begin with a lowercase letter.
The names of abstract data types, structures, typedefs, and enumerated types are to begin with an uppercase letter.
In names which consist of more than one word, the words are written together and each word that follows the first is begun with an uppercase letter.
Do not use identifiers which begin with one or two underscores
(`_
' or `__
').
A name that begins with an uppercase letter is to appear directly after its prefix.
A name that begins with a lowercase letter is to be separated from
its prefix using an underscore (`_
').
A name is to be separated from its suffix using an underscore
(`_
').
Do not use typenames that differ only by the use of uppercase and lowercase letters.
Names should not include abbreviations that are not generally accepted.
A variable with a large scope should have a long name.
Choose variable names that suggest the usage.
Write code in a way that makes it easy to change the prefix for global identifiers.
Encapsulate global variables and constants, enumerated types, and typedefs in a class.
In this chapter, it is important to distinguish between identifiers and names. The name is that part of an identifier that shows its meaning. An identifier consists of a prefix, a name, and a suffix (in that order). The prefix and the suffix are optional. A suffix is only used by tools that generate C++ code, to avoid name collisions with user-written C++ code and is not given further consideration here.
It is recommended identifiers not be extremely long, to reduce the risk for name collisions when using tools that truncate long identifiers.
The Unix command ar
truncates file names that are
longer than 15 characters. Cfront normally modifies generated
C-identifiers that are longer than 31 characters by truncating them
and adding a hash value that is generated from the truncated part of
the string.
The use of two underscores (`__
') in identifiers is
reserved for the compiler's internal use according to the ANSI-C
standard.
Underscores (`_
') are often used in names of library
functions (such as _main
and _exit
). In
order to avoid collisions, do not begin an identifier with an
underscore.
One rule of thumb is that a name which cannot be pronounced is a bad name. A long name is normally better than a short, cryptic name, but the truncation problem must be taken into consideration. Abbreviations can always be misunderstood. Global variables, functions and constants ought to have long enough names to avoid name conflicts, but not too long.
Classes should be named so that "object.function
" is
easy to read and appears to be logical.
There are many class libraries available for purchase and there may be tens of thousands of classes in a large project! Because of this, it is important to be careful that name collisions do not occur. One way of preventing collisions is to have strict rules for assigning names to globally visible objects (such as our use of a prefix). In this way, classes from several different class libraries may be used at the same time.
Names for the following types of objects are to be prefixed:
#define
)The use of prefixes can sometimes be avoided by using a class to limit the scope of the name. Static variables in a class should be used instead of global variables and constants, enumerated data types, and typedefs. Although nested classes may be used in C++, these give rise to too many questions (in connection with the language definition) to be able to recommend their use.
No exceptions.
No exceptions. (At times, an identifier begins with an abbreviation written in uppercase letters, to emphasize the way in which the name is used. Such an abbreviation is considered to be a prefix.)
If the last letter in a word is in uppercase, an underscore is to be used as a word separator.
No exceptions.
No exceptions.
No exceptions.
No exceptions.
No exceptions.
Exception using compound names
const char* functionTitle = "EUA_Special"; int currentIO_Stream = 1; // Last Character in currentIO is in uppercase!
Choice of names
int groupID; // instead of grpID int nameLength; // instead of namLn PrinterStatus resetPrinter; // instead of rstprt
Ambiguous names
void termProcess(); // Terminate process or // terminal process?
Names having numeric characters can cause errors which are difficult to locate.
int I0 = 13; // Names with digits can be int IO = I0; // difficult to read.
Definition of a class in the class library Emc2
class Emc2Class { public: Emc2Class(); // Default constructor // ... private: int id; // ... };
One way to avoid global functions and classes
// Instead of declaring: void Emc2_myFunc1(); void Emc2_myFunc2(); class Emc2MyClass { /* ... */ }; // Encapsulate the functions using an abstract class: class Emc2 { public: static void myFunc1(); static void myFunc2(); class MyClass { /* ... */ }; private: virtual dummy() = 0; // Trick to make the class abstract }; // Now, functions and classes may be accessed by using the scope-operator: Emc2::myFunc1(); Emc2::myFunc2(); Emc2::MyClass myObject;
The public
, protected
, and
private
sections of a class are to be declared in that
order (the public
section is declared before the
protected
section which is declared before the
private
section).
No member functions are to be defined within the class definition.
By placing the public
section first, everything that
is of interest to a user is gathered in the beginning of the class
definition. The protected
section may be of interest to
designers when considering inheriting from the class. The
private
section contains details that should have the
least general interest.
A member function that is defined within a class definition automatically becomes inline. Class definitions are less compact and more difficult to read when they include definitions of member functions. It is easier for an inline member function to become an ordinary member function if the definition of the inline function is placed outside of the class definition. This rule will be in effect at least as long as traditional text editors are used.
A similar problem is that members are private if no access specifier is explicitly given. This problem is avoided by following Rule 20.
No exceptions.
No exceptions.
A class definition in accordance with the style rules
class String : private Object { public: String(); // Default constructor String( const String& s ); // Copy constructor unsigned length() const; // ... protected: int checkIndex( unsigned index ) const; // ... private: unsigned noOfChars; // ... };
No definitions of member functions within the class definition (See Example 1)
// Instead of writing like this: class String { public: int length() const // No !! { return len; } // ... private: int len; }; // Do it this way: class String { public: int length() const; // ... private: int len; }; inline int String::length() const { return len; }
Always provide the return type of a function explicitly.
When declaring functions, the leading parenthesis and the first argument (if any) are to be written on the same line as the function name. If space permits, other arguments and the closing parenthesis may also be written on the same line as the function name. Otherwise, each additional argument is to be written on a separate line (with the closing parenthesis directly after the last argument).
In a function definition, the return type of the function should be written on a separate line directly above the function name.
Always write the left parenthesis directly after a function name.
If no return type is explicitly provided for a function, it is, by
default, an int
. It is recommended to always provide the
return type explicitly, to increase the readability of the code. By
defining the return type on a separate line directly above the
function definition, the function name is more easily seen.
The other recommendations are meant to give a uniform appearance to the code that is written. Until such time as formatting tools are available, programmers should follow these guidelines.
The left parenthesis always directly after the function name
void foo (); // No!! void foo(); // Better
Right and wrong ways of declaring formal arguments for a function (in function definition)
// Right: int myComplicatedFunction( unsigned unsignedValue, int intValue, char* charPointerValue, int* intPointerValue, myClass* myClassPointerValue, unsigned* unsignedPointerValue ); // Wrong: int myComplicatedFunction( unsigned unsignedValue, int intValue, char* charPointerValue, int* intPointerValue, myClass* myClassPointerValue, unsigned* unsignedPointerValue );
Braces ("{}
") which enclose a block are to be placed
in the same column, on separate lines directly before and after the
block.
The placement of braces seems to have been the subject of the greatest debate concerning the appearance of both C and C++ code. We recommend the style which, in our opinion, gives the most readable code. Other styles may well provide more compact code.
The flow control primitives if
, else
,
while
, for
and do
should be
followed by a block, even if it is an empty block.
At times, everything that is to be done in a loop may be easily written on one line in the loop statement itself. It may then be tempting to conclude the statement with a semicolon at the end of the line. This may lead to misunderstanding since, when reading the code, it is easy to miss such a semicolon. It seems to be better, in such cases, to place an empty block after the statement to make completely clear what the code is doing.
Flow control structure without statements
// No block at all - No! while ( /* Something */ ); // Empty block - better! while ( /* Something */ ) { // Empty ! }
The dereference operator `*
' and the address-of
operator `&
' should be directly connected with the
type names in declarations and definitions.
The characters `*
' and `&
' should be
written together with the types of variables instead of with the names
of variables in order to emphasize that they are part of the type
definition. Instead of saying that *i
is an
int
, say that i
is an int*
.
Traditionally, C recommendations indicate that `*
'
should be written together with the variable name, since this reduces
the probability of making a mistake when declaring several variables
in the same declaration statement (the operator `*
' only
applies to the variable on which it operates). Since the declaration
of several variables in the same statement is not recommended,
however, such an advice is unneeded.
*
and &
together
with the type
char* Object::asString() { // Something }; char* userName = 0; int sfBook = 42; int& anIntRef = sfBook;
Declaration of several variables in the same statement
// NOT RECOMMENDED char* i,j; // i is declared pointer to char, while j is declared char
Do not use spaces around `.
' or `->
',
nor between unary operators and operands.
Use the c++ mode in GNU Emacs to format code.
In our opinion, code is more readable if spaces are not used around
the .
or ->
operators. The same applies
to unary operators (those that operate on one operand), since a space
may give the impression that the unary operand is actually a binary
operator.
Ordinary spaces should be used instead of tabs. Since different
editors treat tab characters differently, the work in perfecting a
layout may have been wasted if another editor is later used. Tab
characters can be removed using the UNIX command expand
.
One alternative is to format code using the editor GNU Emacs.
We recommend that everyone use GNU Emacs to format code. Code will then have a uniform appearance regardless of who has written it.
Never specify public
or protected
member
data in a class.
The use of public variables is discouraged for the following reasons:
BankAccount
, in which account_balance
is a
public variable, the value of this variable may be changed by any
user of the class. However, if the variable has been declared
private, its value may be changed only by the member functions of the
class.The use of protected variables in a class are not recommended,
since its variables become visible to its derived classes. The names
of types or variables in a base class may then not be changed since
the derived classes may depend on them. If a derived class, for some
reason, must access data in its base class, one solution may be to
make a special protected interface in the base class, containing
functions which return private data. This solution would not imply
any degradation of performance if the functions are defined
inline
.
The use of structs is also discouraged since these only contain public data. In interfaces with other languages (such as C), it may, however, be necessary to use structs.
In interfaces with other languages (such as C), it may be necessary to use structs having public data.
The correct way to encapsulate data so that future changes are possible.
// Original class: class Symbol {}; class OldSymbol : public Symbol {}; class Priority { public: // returns pd int priority(); // returns symbol class Symbol* getSymbol() const; // ... private: int pd; OldSymbol symbol; }; // Modified class: // The programmer has chosen to change the private data from an int // to an enum. A user of the class `Priority' does not have to change // any code, since the enum return-value from the member function // priority() is automatically converted to an int. class Symbol {}; class NewSymbol : public Symbol {}; enum Priority { low, high, urgent }; class Priority { public: // Interface intact through implicit cast, returns priority_data Priority priority(); // Interface intact, object of new subclass to symbol returned class Symbol* getSymbol() const; // ... private: Priority priority_data; // New representation/name of internal data NewSymbol symbol; };
Access functions are to be inline
.
Forwarding functions are to be inline
.
Constructors and destructors must not be inline
.
The normal reason for declaring a function inline
is
to improve its performance.
Small functions, such as access functions, which return the value
of a member of the class and so-called forwarding functions which
invoke another function should normally be inline
.
Correct usage of inline functions may also lead to reduced size of code.
Warning: functions which invoke other inline functions often become too complex for the compiler to be able to make them inline despite their apparent smallness.
This problem is especially common with constructors and destructors. A constructor always invokes the constructors of its base classes and member data before executing its own code. Always avoid inline constructors and destructors!
Friends of a class should be used to provide additional functions that are best kept outside of the class.
Operations on an object are sometimes provided by a collection of classes and functions.
A friend is a nonmember of a class, that has access to the nonpublic members of the class. Friends offer an orderly way of getting around data encapsulation for a class. A friend class can be advantageously used to provide functions which require data that is not normally needed by the class.
Suppose there is a list class which needs a pointer to an internal list element in order to iterate through the class. This pointer is not needed for other operations on the list. There may then be reason, such as obtaining smaller list objects, for an list object not to store a pointer to the current list element and instead to create an iterator, containing such a pointer, when it is needed.
One problem with this solution is that the iterator class normally does not have access to the data structures which are used to represent the list (since we also recommend private member data).
By declaring the iterator class as a friend
, this
problem is avoided without violating data encapsulation.
Friends are good if used properly. However, the use of many friends can indicate that the modularity of the system is poor.
const
Member FunctionsA member function that does not affect the state of an object (its
instance variables) is to be declared const
.
If the behaviour of an object is dependent on data outside the
object, this data is not to be modified by const
member
functions.
Member functions declared as const
may not modify
member data and are the only functions which may be invoked on a
const
object. (Such an object is clearly unusable
without const
methods.) A const
declaration
is an excellent insurance that objects will not be modified (mutated)
when they should not be. A great advantage that is provided by C++ is
the ability to overload functions with respect to their
const
-ness. (Two member functions may have the same name
where one is const
and the other is not.)
Non-const
member functions are sometimes invoked as
so-called 'lvalues' (as a location value at which a value may be
stored). A const
member function may never be invoked as
an 'lvalue'.
The behaviour of an object can be affected by data outside the
object. Such data must not be modified by a const
member
function.
At times, it is desirable to modify data in a const
object (such a having a cache of data for performance reasons). In
order to avoid explicit type conversions from a const
type to a non-const
type, the only way is to store the
information outside the object. (See example 55.)
This type of data should be seen as external data which does not
affect the behaviour of the class.
No exceptions.
No exceptions.
const
-declared access functions to internal data in a class
class SpecialAccount : public Account { public: int insertMoney(); // int getAmountOfMoney(); No! Forbids ANY constant object to // access the amount of money. int getAmountOfMoney() const; // Better! // ... private: int moneyAmount; };
Overloading an operator/function with respect to
const
-ness
#include <iostream.h> #include <string.h> static unsigned const cSize = 1024; class InternalData {}; class Buffer { public: Buffer( char* cp ); // Inline functions in this class are written compactly so the example // may fit on one page. THIS is NOT to be done in practice (See Rule 21). // A. non-const member functions: result is an lvalue char& operator[]( unsigned index ) { return buffer[index]; } InternalData& get() { return data; } // B. const member functions: result is not an lvalue char operator[]( unsigned index ) const { return buffer[index]; } const InternalData& get() const { return data; } private: char buffer[cSize]; InternalData data; }; inline Buffer::Buffer( char* cp ) { strncpy( buffer , cp , sizeof( buffer ) ); } main() { const Buffer cfoo = "peter"; // This is a constant buffer Buffer foo = "mary"; // This buffer can change foo[2]='c'; // calls char& Buffer::operator[](unsigned) cfoo[2] = 'c' // ERROR: cfoo[2] is not an lvalue. // cfoo[2] means that Buffer::operator[](unsigned) const is called. cout << cfoo[2] << ":" << foo[2] << endl; // OK! Only rvalues are needed foo.get() = cfoo.get(); cfoo.get() = foo.get(); // ERROR: cfoo.get() is not an lvalue }
A class which uses "new
" to allocate instances managed
by the class, must define a copy constructor.
All classes which are used as base classes and which have virtual functions, must define a virtual destructor.
Avoid the use of global objects in constructors and destructors.
A copy constructor is recommended to avoid surprises when an object is initialized using an object of the same type. If an object manages the allocation and deallocation of an object on the heap (the managing object has a pointer to the object to be created by the class' constructor), only the value of the pointer will be copied. This can lead to two invocations of the destructor for the same object (on the heap), probably resulting in a run-time error.
The corresponding problem exists for the assignment operator
(`=
'). See 7.6: Assignment
Operators.
If a class, having virtual functions but without virtual
destructors, is used as a base class, there may be a surprise if
pointers to the class are used. If such a pointer is assigned to an
instance of a derived class and if delete
is then used on
this pointer, only the base class' destructor will be invoked. If the
program depends on the derived class' destructor being invoked, the
program will fail.
In connection with the initialization of statically allocated objects, it is not certain that other static objects will be initialized (for example, global objects). This is because the order of initialization of static objects which is defined in various compilation units, is not defined in the language definition. There are ways of avoiding this problem, all of which require some extra work.
You must know what you are doing if you invoke virtual functions from a constructor in the class. If virtual functions in a derived class are overridden, the original definition in the base class will still be invoked by the base class' constructor. Override, then, does not always work when invoking virtual functions in constructors. See Example 30.
Sometimes, it is desired to let objects in a class share a data area. In such a case, it is not necessary to define a copy constructor. Instead, it is necessary to make sure that this data area is not deallocated as long as there are pointers to it.
No exceptions.
Definition of a "dangerous" class not having a copy constructor
#include <string.h> class String { public: String( const char* cp = ""); // Constructor ~String(); // Destructor // ... private: char* sp; // ...  }
"Safe" class having copy constructor and default constructor
#include <string.h> class String { public: String( const char* cp = ""); // Constructor String( const String& sp ); // Copy constructor ~String(); // Destructor // ... private: char* sp; // ... }; String::String( const char* cp ) : sp( new char[strlen(cp)] ) // Constructor { strcpy(sp,cp); } String::String( const String& stringA ) : sp( new char[strlen(stringA.sp)] ) { strcpy(sp,stringA.sp); } String::~String() // Destructor { delete sp; } // "Safe" String class void main() { String w1; String w2 = w1; // SAFE COPY: String::String( const String& ) CALLED. }
Definitions of classes not having virtual destructors
class Fruit { public: ~Fruit(); // Forgot to make destructor virtual!! // ... }; class Apple : public Fruit { public: ~Apple(); // Destructor // ... }; // "Dangerous" usage of pointer to base class class FruitBasket { public: FruitBasket(); // Create FruitBasket ~FruitBasket(); // Delete all fruits // ... void add(Fruit*); // Add instance allocated on the free store // ... private: Fruit* storage[42]; // Max 42 fruits stored int numberOfStoredFruits; }; void FruitBasket::add(Fruit* fp) { // Store pointer to fruit storage[numberOfStoredFruits++] = fp; } FruitBasket::FruitBasket() : numberOfStoredFruits(0) { } FruitBasket::~FruitBasket() { while (numberOfStoredFruits>0) { delete storage[--numberOfStoredFruits]; // Only Fruit::~Fruit is called !! } }
Dangerous use of static objects in constructors
// Hen.hh class Egg; class Hen { public: Hen(); // Default constructor ~Hen(); // Destructor // ... void makeNewHen(Egg*); // ... }; // Egg.hh class Egg { }; extern Egg theFirstEgg; // defined in Egg.cc // FirstHen.hh class FirstHen : public Hen { public: FirstHen(); // Default constructor // ... }; extern FirstHen theFirstHen; // defined in FirstHen.cc // FirstHen.cc FirstHen theFirstHen; // FirstHen::FirstHen() called FirstHen::FirstHen() { // The constructor is risky because theFirstEgg is a global object // and may not yet exist when theFirstHen is initialized. // Which comes first, the chicken or the egg ? makeNewHen(&theFirstEgg); }
One way of ensuring that global objects have been initialized
// WARNING!!! THIS CODE IS NOT FOR BEGINNERS !!! // PortSetup.hh class PortSetup { public: PortSetup(); // Constructor: initializes flag void foo(); // Only works correctly if flag is 42 private: int flag; // Always initialized to 42 }; extern PortSetup portSetup; // Must be initialized before use // Create one instance of portSetupInit in each translation unit // The constructor for portSetupInit will be called once for each // translation unit. It initializes portSetup by using the placement // syntax for the "new" operator. static class PortSetupInit { public: PortSetupInit(); // Default constructor private: static int isPortSetup; } portSetupInit; // PortSetup.cc #include "PortSetup.hh" #include <new.h> // ... PortSetupInit::PortSetupInit() // Default constructor { if (!isPortSetup) { new (&portSetup) PortSetup; isPortSetup = 1; } }
Override of virtual functions does not work in the base class' constructors
class Base { public: Base(); // Default constructor virtual void foo() { cout << "Base::foo" << endl; } // ... }; Base::Base() { foo(); // Base::foo() is ALWAYS called. } // Derived class overrides foo() class Derived : public Base { public: virtual void foo() { cout << "Derived::foo" << endl; } //foo is overridden // ... }; main() { Derived d; // Base::foo() called when the Base-part of // Derived is constructed. }
A class which uses "new
" to allocate instances managed
by the class, must define an assignment operator.
An assignment operator which performs a destructive action must be protected from performing this action on the object upon which it is operating.
An assignment operator ought to return a const
reference to the assigning object.
An assignment is not inherited like other operators. If an assignment operator is not explicitly defined, then one is automatically defined instead. Such an assignment operator does not perform bit-wise copying of member data; instead, the assignment operator (if defined) for each specific type of member data is invoked. Bit-wise copying is only performed for member data having primitive types.
One consequence of this is that bit-wise copying is performed for member data having pointer types. If an object manages the allocation of the instance of an object pointed to by a pointer member, this will probably lead to problems: either by invoking the destructor for the managed object more than once or by attempting to use the deallocated object. See also Rule 25.
If an assignment operator is overloaded, the programmer must make certain that the base class' and the members' assignment operators are run.
A common error is assigning an object to itself (a =
a
). Normally, destructors for instances which are allocated on
the heap are invoked before assignment takes place. If an object is
assigned to itself, the values of the instance variables will be lost
before they are assigned. This may well lead to strange run-time
errors. If a = a
is detected, the assigned object should
not be changed.
If an assignment operator returns "void
", then it is
not possible to write a = b = c
. It may then be tempting
to program the assignment operator so that it returns a reference to
the assigning object. Unfortunately, this kind of design can be
difficult to understand. The statement (a = b) = c
can
mean that a
or b
is assigned the value of
c
before or after a
is assigned the value of
b
. This type of code can be avoided by having the
assignment operator return a const
reference to the
assigned object or to the assigning object. Since the returned object
cannot be placed on the left side of an assignment, it makes no
difference which of them is returned (that is, the code in the above
example is no longer correct).
Sometimes, it is desirable to allow objects in a class to share a data area. In such cases, it is not necessary to define an assignment operator. Instead, it is necessary to make sure that the shared data area is no deallocated as long as there are pointers to it.
No exceptions.
Incorrect and correct return values from an assignment operator
void MySpecialClass::operator=( const MySpecialClass& msp ); // Well ...? MySpecialClass& MySpecialClass::operator=( const MySpecialClass& msp ); // No! const MySpecialClass& MySpecialClass::operator=( const MySpecialClass& msp ); // Recommended
Definition of a class with an overloaded assignment operator
class DangerousBlob { public: const DangerousBlob& operator=( const DangerousBlob& dbr ); // ... private: char* cp; }; // Definition of assignment operator const DangerousBlob& DangerousBlob::operator=( const DangerousBlob& dbr ) { if ( this != &dbr ) // Guard against assigning to the "this" pointer { delete cp; // Disastrous if this == &dbr } // ... }
Use operator overloading sparingly and in a uniform manner.
When two operators are opposites (such as ==
and
!=
), it is appropriate to define both.
Operator overloading has both advantages and disadvantages. One advantage is that code which uses a class with overloaded operators can be written more compactly (more readably). Another advantage is that the semantics can be both simple and natural. One disadvantage in overloading operators is that it is easy to misunderstand the meaning of an overloaded operator (if the programmer has not used natural semantics). The extreme case, where the plus-operator is re-defined to mean minus and the minus-operator is re-defined to mean plus, probably will not occur very often, but more subtle cases are conceivable.
Designing a class library is like designing a language! If you use operator overloading, use it in a uniform manner; do not use it if it can easily give rise to misunderstanding.
If the operator !=
has been designed for a class, then
a user may well be surprised if the operator ==
is not
defined as well.
A public member function must never return a non-const
reference or pointer to member data.
A public member function must never return a non-const
reference or pointer to data outside an object, unless the object
shares the data with other objects.
By allowing a user direct access to the private member data of an object, this data may be changed in ways not intended by the class designer. This may lead to reduced confidence in the designer's code: a situation to be avoided.
A worse risk is having pointers which point to deallocated memory. Rule 29 and Rule 30 attempt to avoid this situation.
Note that we do not forbid the use of protected member functions
which return a const
reference or pointer to member data.
If protected access functions are provided, the problems described in
7.1 are avoided.
No exceptions.
No exceptions.
Never return a non-const
reference to member data from
a public function.
class Account { public: Account( int myMoney ) : moneyAmount( myMoney ) {}; const int& getSafeMoney() const { return moneyAmount; } int& getRiskyMoney() const { return moneyAmount; } // No! // ... private: int moneyAmount; }; Account myAcc(10); // I'm a poor lonesome programmer a long way from home myAcc.getSafeMoney() += 1000000; // Compilation error: assignment to constant myAcc.getRiskyMoney() += 1000000; // myAcc::moneyAmount = 1000010 !!
Avoid inheritance for parts-of relations.
Give derived classes access to class type member data by declaring
protected
access functions.
A common mistake is to use multiple inheritance for parts-of relations (when an object consists of several other objects, these are inherited instead of using instance variables. This can result in strange class hierarchies and less flexible code. In C++ there may be an arbitrary number of instances of a given type; if inheritance is used, direct inheritance from a class may only be used once.
A derived class often requires access to base class member data in
order to create useful member functions. The advantage in using
protected member functions is that the names of base class member data
are not visible in the derived classes and thus may be changed. Such
access functions should only return the values of member data
(read-only access). This is best done by simply invoking
const
functions for the member data.
The guiding assumption is that those who use inheritance know enough about the base class to be able to use the private member data correctly, while not referring to this data by name. This reduces the coupling between base classes and derived classes.
Do not attempt to create an instance of a class template using a type that does not define the member functions which the class template, according to its documentation, requires.
Take care to avoid multiple definition of overloaded functions in conjunction with the instantiation of a class template.
It is not possible in C++ to specify requirements for type arguments for class templates and function templates. This may imply that the type chosen by the user, does not comply with the interface as required by the template. For example, a class template may require that a type argument have a comparison operator defined.
Another problem with type templates can arise for overloaded
functions. If a function is overloaded, there may be a conflict if
the element type appears explicitly in one of these. After
instantiation, there may be two functions which, for example, have the
type int
as an argument. The compiler may complain about
this, but there is a risk that the designer of the class does not
notice it. In cases where there is a risk for multiple definition of
member functions, this must be carefully documented.
Problem when using parameterized types (Cfront 3.0 or other template compiler)
template <class ET> class Conflict { public: void foo( int a ); void foo( ET a ); // What if ET is an int or another integral type? // The compiler will discover this, but ... };
Unless otherwise stated, the following rules also apply to member functions.
Do not use unspecified function arguments (ellipsis notation).
Avoid functions with many arguments.
If a function stores a pointer to an object which is accessed via an argument, let the argument have the type pointer. Use reference arguments in other cases.
Use constant references
(const &
) instead of call-by-value, unless using a
pre-defined data type or a pointer.
The best known function which uses unspecified arguments is
printf()
. The use of such functions is not advised since
the strong type checking provided by C++ is thereby avoided. Some of
the possibilities provided by unspecified function arguments can be
attained by overloading functions and by using default arguments.
Functions having long lists of arguments look complicated, are difficult to read, and can indicate poor design. In addition, they are difficult to use and to maintain.
By using references instead of pointers as function arguments, code can be made more readable, especially within the function. A disadvantage is that it is not easy to see which functions change the values of their arguments. Member functions which store pointers which have been provided as arguments should document this clearly by declaring the argument as a pointer instead of as a reference. This simplifies the code, since it is normal to store a pointer member as a reference to an object.
One difference between references and pointers is that there is no null-reference in the language, whereas there is a null-pointer. This means that an object must have been allocated before passing it to a function. The advantage with this is that it is not necessary to test the existence of the object within the function.
C++ invokes functions according to call-by-value. This means that
the function arguments are copied to the stack via invocations of copy
constructors, which, for large objects, reduces performance. In
addition, destructors will be invoked when exiting the function.
const &
arguments mean that only a reference to the
object in question is placed on the stack (call-by-reference) and that
the object's state (its instance variables) cannot be modified. (At
least some const
member functions are necessary for such
objects to be at all useful.)
No exceptions.
References instead of pointers
// Unnecessarily complicated use of pointers void addOneComplicated( int* integerPointer ) { *integerPointer += 1; } addOneComplicated( &j ); // Write this way instead: void addOneEasy( int& integerReference ) { integerReference += 1; } addOneEasy( i );
Different mechanisms for passing arguments
// a. A copy of the argument is created on the stack. // The copy constructor is called on entry, // and the destructor is called at exit from the function. // This may lead to very inefficient code. void foo1( String s ); String a; foo1( a ); // call-by-value // b. The actual argument is used by the function // and it can be modified by the function. void foo2( String& s ); String b; foo2( b ); // call-by-reference // c. The actual argument is used by the function // but it cannot be modified by the function. void foo3( const String& s ); String c; foo3( c ); // call-by-constant-reference // d. A pointer to the actual argument is used by the function. // May lead to messy syntax when the function uses the argument. void foo4( const String* s ); String d; foo4( &d ); // call-by-constant-pointer
When overloading functions, all variations should have the same semantics (be used for the same purpose).
Overloading of functions can be a powerful tool for creating a family of related functions that only differ as to the type of data provided as arguments. If not used properly (such as using functions with the same name for different purposes), they can, however, cause considerable confusion.
Example of the proper usage of function overloading
class String { public: // Used like this: // ... // String x = "abc123"; int contains( const char c ); // int i = x.contains( 'b' ); int contains( const char* cs ); // int j = x.contains( "bc1" ); int contains( const String& s ); // int k = x.contains( x ); // ... };
The names of formal arguments to functions are to be specified and are to be the same both in the function declaration and in the function definition.
The names of formal arguments may be specified in both the function declaration and the function definition in C++, even if these are ignored by the compiler in the declaration. Providing names for function arguments is a part of the function documentation. The name of an argument may clarify how the argument is used, reducing the need to include comments in, for example, a class definition. It is also easier to refer to an argument in the documentation of a class if it has a name.
No exceptions!
Declaration of formal arguments
int setPoint( int, int ); // No ! int setPoint( int x, int y ); // Good int setPoint( int x, int y ) { // ... }
Always specify the return type of a function explicitly.
A public function must never return a reference or a pointer to a local variable.
Functions, for which no return type is explicitly declared,
implicitly receive int
as the return type. This can be
confusing for a beginner, since the compiler gives a warning for a
missing return type. Because of this, functions which return no value
should specify void as the return type.
If a function returns a reference or a pointer to a local variable, the memory to which it refers will already have been deallocated, when this reference or pointer is used. The compiler may or may not give a warning for this.
No exceptions!
No exceptions!
Functions which return no value should be specified as having the
return type void
.
void strangeFunction( const char* before, const char* after ) { // ... }
Do not use the preprocessor
directive #define
to obtain more efficient code; instead,
use inline functions.
Use inline functions when they are really needed.
See also 7.2.
Inline functions have the advantage of often being faster to execute than ordinary functions. The disadvantage in their use is that the implementation becomes more exposed, since the definition of an inline function must be placed in an include file for the class, while the definition of an ordinary function may be placed in its own separate file.
A result of this is that a change in the implementation of an
inline function can require comprehensive re-compiling when the
include file is changed. This is true for traditional file-based
programming environments which use such mechanisms as
make
for compilation.
The compiler is not compelled to actually make a function inline. The decision criteria for this differ from one compiler to another. It is often possible to set a compiler flag so that the compiler gives a warning each time it does not make a function inline (contrary to the declaration). "Outlined inlines" can result in programs that are both unnecessarily large and slow.
It may be appropriate to separate inline definitions from class definitions and to place these in a separate file.
No exceptions
Inline functions are better than macros
// Example of problems with #define "functions" #define SQUARE(x) ((x)*(x)) int a = 2; int b = SQUARE(a++); // b = (2 * 3) = 6 // Inline functions are safer and easier to use than macros if you // need an ordinary function that would have been unacceptable for // efficiency reasons. // They are also easier to convert to ordinary functions later on. inline int square( int x ) { return ( x * x ); }; int c = 2; int d = square( c++ ); // d = ( 2 * 2 ) = 4
Minimize the number of temporary objects that are created as return values from functions or as arguments to functions.
Temporary objects are often created when objects are returned from functions or when objects are given as arguments to functions. In either case, a constructor for the object is first invoked; later, a destructor is invoked. Large temporary objects make for inefficient code. In some cases, errors are introduced when temporary objects are created. It is important to keep this in mind when writing code. It is especially inappropriate to have pointers to temporary objects, since the lifetime of a temporary object is undefined. (See 18.7.)
Temporary objects and one way of eliminating them
class BigObject { double big[123456]; }; // Example of a very inefficient function with respect to temporary objects: BigObject slowTransform( BigObject myBO ) { // When entering slowTransform(), myBO is a copy of the function argument // provided by the user. -> A copy constructor for BigObject is executed. // ... Transform myBO in some way return myBO; // Transformed myBO returned to the user } // When exiting slowTransform(), a copy of myBO is returned to the // user -> copy-constructor for BigObject is executed, again. // Much more efficient solution: BigObject& fastTransform( BigObject& myBO ) { // When entering fastTransform(), myBO is the same object as the function // argument provided by the user. -> No copy-constructor is executed. // Transform myBO in some way return myBO; // Transformed myBO is returned to the user. } // When exiting fastTransform(), the very same myBO is returned // to the user. -> No copy constructor executed. void main() { BigObject BO; BO = slowTransform( BO ); BO = fastTransform( BO ); // Same syntax as slowTransform() !! }
Avoid long and complex functions.
Long functions have disadvantages:
Complex functions are difficult to test. If a function consists of
15 nested if
statements, then there are 2**15 (or 32768)
different branches to test in a single function.
Constants are to be defined
using const
or enum
; never using
#define
.
Avoid the use of numeric values in code; use symbolic values instead.
The preprocessor performs a textual substitution for macros in the
source code which is then compiled. This has a number of negative
consequences. For example, if a constant has been defined using
#define
, the name of the constant is not recognized in
many debuggers. If the constant is represented by an expression, this
expression may be evaluated differently for different instantiations,
depending on the scope of the name. In addition, macros are, at
times, incorrectly written.
Numerical values in code ("Magic Numbers") should be viewed with
suspicion. They can be the cause of difficult problems if and when it
becomes necessary to change a value. A large amount of code can be
dependent on such a value never changing, the value can be used at a
number of places in the code (it may be difficult to locate all of
them), and values as such are rather anonymous (it may be that every
`2
' in the code should not be changed to a
`3
').
From the point of view of portability, absolute values may be the cause of more subtle problems. The type of a numeric value is dependent on the implementation. Normally, the type of a numeric value is defined as the smallest type which can contain the value.
No exceptions.
Certain numerical values have a well established and clear meaning in
a program. For example, '1
' and '0
' are
often used to represent `true
' and `false
',
respectively. These may be used directly in code without being
considered to be "Magic".
Different ways of declaring constants.
// Constants using macros #define BUFSIZE 7 // No type checking // Constants using const const int bufSize = 7; // Type checking takes place // Constants using enums enum SIZE { BufSize = 7 }; // Type checking takes place
Declaration of const defined in another file
extern const char constantCharacter; extern const String fileName;
Variables are to be declared with the smallest possible scope.
Each variable is to be declared in a separate declaration statement.
Every variable that is declared is to be given a value before it is used.
If possible, always use initialization instead of assignment.
A variable ought to be declared with the smallest possible scope to improve the readability of the code and so that variables are not unnecessarily allocated. When a variable that is declared at the beginning of a function is used somewhere in the code, it is not easy to directly see the type of the variable. In addition, there is a risk that such a variable is inadvertently hidden if a local variable, having the same name, is declared in an internal block.
Many local variables are only used in special cases which seldom occur. If a variable is declared at the outer level, memory will be allocated even if it is not used. In addition, when variables are initialized upon declaration, more efficient code is obtained than if values are assigned when the variable is used.
A variable must always be initialized before use. Normally, the
compiler gives a warning if a variable is undefined. It is then
sufficient to take care of such cases. Instances of a class are
usually initialized even if no arguments are provided in the
declaration (the empty constructor is invoked). To declare a variable
that has been initialized in another file, the keyword
extern
is always used.
By always initializing variables, instead of assigning values to them before they are first used, the code is made more efficient since no temporary objects are created for the initialization. For objects having large amounts of data, this can result in significantly faster code.
No exceptions.
No exceptions.
No exceptions.
In certain special cases, a variable is assigned the value of a complicated expression; it may then be unnecessary to give the variable an initial value. See Example 44.
Initialization instead of Assignment
// Do not do this! // int i; // ... 1022 lines of code // i = 10; int j = 10; // Better class Special // Array of this class is used to initialize { // MyClass::complicated public: Special(); // Default constructor int isValid() const; int value() const; }; const int Magic = 1066; Special specialInit[Magic]; class MyClass { public: MyClass( const char* init ); // Constructor // ... private: String privateString; int complicated; }; // Do not do this! Inefficient code. // Empty constructor + assignment operator called for privateString // // MyClass::MyClass( const char* init ) // { // privateString = init; // ... // } MyClass::MyClass( const char* init ) : privateString( init ) // Better { // Special case - complicated expression for( int i = 0; i < Magic; i++ ) // No! You should enclose "for" if ( specialInit[i].isValid() ) // loops in braces! See Rec. 25! { complicated = specialInit[i].value(); break; } }
Do not compare a pointer to NULL
or assign
NULL
to a pointer; use 0
instead.
Pointers to pointers should whenever possible be avoided.
Use a typedef
to
simplify program syntax when declaring function pointers.
According to the ANSI-C standard, NULL
is defined
either as (void*)0
or as 0
. If this
definition remains in ANSI-C++, problems may arise. If
NULL
is defined to have the type void*, it cannot be
assigned an arbitrary pointer without an explicit type conversion.
For this reason, we recommend comparisons with 0
at least
until the ANSI-C++ committee has made a decision.
Pointers to pointers normally ought not be used. Instead, a class
should be declared, which has a member variable of the pointer type.
This improves the readability of the code and encourages data
abstraction. By improving the readability of code, the probability of
failure is reduced. One exception to this rule is represented by
functions which provide interfaces to other languages (such as C).
These are likely to only allow pre-defined data types to be used as
arguments in the interface, in which case pointers to pointers are
needed. Another example is the second argument to the
main
function, which must have the type
char*[]
.
A function which changes the value of a pointer that is provided as
an argument, should declare the argument as having the type reference
to pointer (e.g. char*&
). See Rec. 42!
typedef
is a good way of making code more easily
maintainable and portable. See chapter 18.1, Port. Rec. 1. Another reason to use
typedef
is that the readability of the code is improved.
If pointers to functions are used, the resulting code can be almost
unreadable. By making a type declaration for the function type, this
is avoided.
Function pointers can be used as ordinary functions; they do not need to be dereferenced.
No exceptions.
Different comparisons of pointers
char* sp = new char[100]; if ( !sp ) cout << "New failed!" << endl; // No! if ( sp == 0 ) cout << "New failed!" << endl; // Best if ( sp == NULL ) cout << "New failed!" << endl; // ERROR sometimes !!!
Pointers to pointers are often unnecessary
#include <iostream.h> void print_mij(int** m, int dim1, int dim2) { for (int i = 0; i < dim1; i++) { for (int j = 0; j < dim2; j++ ) cout << " " << ((int*)m)[i*dim2+j]; cout << endl; } } // Could be written as: class Int_Matrix { public: Int_Matrix(int dim1, int dim2); int value(int,int) const; int dim1() const; int dim2() const; // .. }; void print_Mij(Int_Matrix m) { for (int i = 0; i < m.dim1(); i++) { for (int j = 0; j < m.dim2(); j++ ) cout << " " << m.value(i,j); cout << endl; } }
Complicated declarations
// func1 is a function: int -> (function : const char* -> int) // i.e. a function having one argument of type int and returning // a pointer to a function having one argument of type const char* // and returning an int. int (*func1(int))(const char*); // func1 of the same type as func2 typedef int FTYPE(const char*); FTYPE* func2(int); int (*(*func1p)(int))(const char*) = func2; // Realistic example from signal.h void (*signal(int,void (*)(int)))(int);
Syntax simplification of
function pointers using a typedef
#include <math.h> // Ordinary messy way of declaring pointers to functions: // double ( *mathFunc ) ( double ) = sqrt; // With a typedef, life is filled with happiness (chinese proverb): typedef double MathFuncType( double ); MathFuncType* mathFunc = sqrt; void main() { // You can invoke the funktion in an easy or complicated way double returnValue1 = mathFunc( 23.0 ); // Easy way double returnValue2 = ( *mathFunc )( 23.0 ); // No! Correct, but complicated }
Never use explicit type conversions (casts).
Do not write code which depends on functions that use implicit type conversions.
Never convert pointers to objects of a derived class to pointers to objects of a virtual base class.
Never convert a const
to a non-const
.
A type conversion may either be explicit or implicit, depending on whether it is ordered by the programmer or by the compiler. Explicit type conversions (casts) are used when a programmer want to get around the compiler's typing system; for success in this endeavour, the programmer must use them correctly. Problems which the compiler avoids may arise, such as if the processor demands that data of a given type be located at certain addresses or if data is truncated because a data type does not have the same size as the original type on a given platform. Explicit type conversions between objects of different types lead, at best, to code that is difficult to read.
Explicit type conversions (casts) can be motivated if a base class pointer to a derived class pointer is needed. This happens when, for example, a heterogeneous container class is used to implement a container class to store pointers to derived class objects. This new class can be made "type-safe" if the programmer excludes other objects than derived class pointers from being stored. In order for this implementation to work, it is necessary that the base class pointers are converted to derived class pointers when they are removed from the heterogeneous container class.
The above reason for using explicit casts will hopefully disappear when templates are introduced into C++.
It is sometimes said that explicit casts are to object-oriented programming, what the goto statement was to structured programming.
There are two kinds of implicit type conversions: either there is a conversion function from one type to another, written by the programmer, or the compiler does it according to the language standard. Both cases can lead to problems.
C++ is lenient concerning the variables that may be used as arguments to functions. If there is no function which exactly matches the types of the arguments, the compiler attempts to convert types to find a match. The disadvantage in this is that if more than one matching function is found, a compilation error will be the result. Even worse is that existing code which the compiler has allowed in other contexts, may contain errors when a new implicit type conversion is added to the code. Suddenly, there may be more than one matching function.
Another unpredictable effect of implicit type conversions is that temporary objects are created during the conversion. This object is then the argument to the function; not the original object. The language definition forbids the assignment of temporary objects to non-constant references, but most compilers still permit this. In most cases, this can mean that the program does not work properly. Be careful with constructors that use only one argument, since this introduces a new type conversion which the compiler can unexpectedly use when it seems reasonable in a given situation.
Virtual base classes give rise to other type conversion problems. It is possible to convert a pointer, to an instance of a class which has a virtual base class, to a pointer to an object of that virtual base class. The opposite conversion is not allowed, i.e. the type conversion is not reversible. For this reason, we do not recommend the conversion of a derived class pointer to a virtual base class pointer.
In order to return a non-const
temporary object, it
sometimes happens that an explicit type conversion is used to convert
const
member data to non-const
. This is bad
practice that should be avoided, primarily because it should be
possible for a compiler to allocate constants in ROM (Read Only
Memory).
An explicit type conversion (cast) is preferable to a doubtful implicit type conversion.
Explicit type conversions may be used to convert a pointer to a base class to a pointer of a derived class within a type-safe container class that is implemented using a heterogeneous container class.
Explicit type conversion must be used to convert an anonymous bit-stream to an object. This situation occurs when unpacking a message in a message buffer. Generally, explicit type conversions are needed for reading an external representation of an object.
At times it is desirable to have constructors that use only one argument. By performing an explicit type conversion, the correctness of the code does not depend on the addition. See the Exception to Rule 22!
If a virtual base class is to contain a pure virtual function which converts a virtual base class pointer to a derived class pointer, this can be made to work by defining the function in the derived class. Note that this implies that all derived classes must be known in the virtual base class. See Example 52!
No exceptions. Use pointers to data allocated outside the class, when necessary. See Example 54 and Example 55.
Constructors with a single argument that may imply dangerous type conversions
class String { public: String( int length ); // Allocation constructor // ... }; // Function that receives an object of type String as an argument void foo( const String& aString ); // Here we call this function with an int as argument int x = 100; foo( x ); // Implicit conversion: foo( String( x ) );
A use of implicit type conversion
// String.hh class String { public: String( char* cp ); // Constructor operator const char* () const; // Conversion operator to const char* // ... }; void foo( const String& aString ); void bar( const char* someChars ); // main.cc main() { foo( "hello" ); // Implicit type conversion char* -> String String peter = "pan"; bar( peter ); // Implicit type conversion String -> const char* }
When implicit type conversion gives unpleasant results
// This function looks bulletproof, but it isn't. // Newer versions of compilers should flag this as an error. void mySwap( int& x, int& y ) { int temp = x; x = y; y = temp; } int i = 10; unsigned int ui = 20; mySwap( i, ui ); // What really happens here is: // int T = int( ui ); // Implicit conversion // mySwap( i, T ); // ui is of course not changed! // Fortunately, the compiler warns for this !
Conversion of derived class pointer to a virtual base class pointer is irreversible
class VirtualBase { public: virtual class Derived* asDerived() = 0; }; class Derived : virtual public VirtualBase { public: virtual Derived* asDerived(); }; Derived* Derived::asDerived() { return this; } void main() { Derived d; Derived* dp = 0; VirtualBase* vp = (VirtualBase*)&d; dp = (Derived*)vp; // ERROR! Cast from virtual base class pointer dp = vp->asDerived(); // OK! Cast in function asDerived }
Addition which leads to a compile-time error
// String.hh class String { public: String( char* cp ); // Constructor operator const char* () const; // Conversion operator to const char* // ... }; void foo( const String& aString ); void bar( const char* someChars ); // Word.hh class Word { public: Word( char* cp ); // Constructor // ... }; // Function foo overloaded void foo( const Word& aWord ); // ERROR: foo( "hello" ) MATCHES BOTH: // void foo( const String& ); // AND void foo( const Word& ); //main.cc main() { foo( "hello" ); // Error ambiguous type conversion ! String peter = "pan"; bar( peter ); // Implicit type conversion String -> const char* }
For more efficient execution, remove
const
-ness when storing intermediate results
// This is code is NOT recommended #include <math.h> class Vector { public: Vector(int, const int []); // Constructor double length() const; // length = sqrt(array[1]*array[1] + ... ) void set(int x, int value); // ... private: int size; int* array; double lengthCache; // to cache calculated length int hasChanged; // is it necessary to re-calculate length ? }; double Vector::length() const { if (hasChanged) // Do we need to re-calculate length { ((Vector*)this)->hasChanged=0; // No! Cast away const double quadLength = 0; for ( int i = 0; i < size; i++ ) { quadLength += pow(array[i],2); } ((Vector*)this)->lengthCache = sqrt(quadLength); // No! Cast away const } return lengthCache; } void Vector::set( int nr, int value ) { if ( nr >= size ) error( "Out Of Bounds"); array[nr]=value; hasChanged = 1; }
Alternative to removing const
-ness for more efficient execution
// This is code is safer than Example 54 but could be inefficient #include <math.h> class Vector { public: Vector(int, const int []); // Constructor double length() const; // length = sqrt(array[1]*array[1] + ... ) void set(int x, int value); // ... private: int size; int* array; double* lengthCache; // to cache length in int* hasChanged; // is it necessary to re-calculate length ? }; Vector::Vector(int sizeA, const int arrayA[]) : size(sizeA), array( new int[sizeA] ), hasChanged(new int(1)), lengthCache(new double) { for ( int i = 0; i < size; i++ ) { array[i] = arrayA[i]; } } Vector::~Vector() // Destructor { delete array; delete hasChanged; delete lengthCache; } // Continue on next page ! double Vector::length() const { if (hasChanged) // Do we need to re-calculate length ? { *hasChanged=0; double quadLength = 0; for ( int i = 0; i < size; i++ ) { quadLength += pow(array[i],2); } *lengthCache = sqrt(quadLength); } return lengthCache; } void Vector::set( int nr, int value ) { if ( nr >= size ) error( "Out Of Bounds"); array[nr]=value; *hasChanged = 1; }
The code following a case
label must always be
terminated by a break
statement.
A switch
statement must always contain a
default
branch which handles unexpected cases.
Never use goto
.
The choice of loop construct (for
, while
or do
-while
) should depend on the specific
use of the loop.
Always use unsigned
for variables which cannot
reasonably have negative values.
Always use inclusive lower limits and exclusive upper limits.
Avoid the use of continue
.
Use break
to exit a loop if this avoids the use of flags.
Do not write logical expressions of the type if(test)
or if(!test)
when test
is a pointer.
Each loop construct has a specific usage. A for
loop
is used only when the loop variable is increased by a constant amount
for each iteration and when the termination of the loop is determined
by a constant expression. In other cases, while
or
do
-while
should be used. When the
terminating condition can be evaluated at the beginning of the loop,
while
should be used; do
-while
is used when the terminating condition is best evaluated at the end of
the loop.
Goto breaks the control flow and can lead to code that is difficult
to comprehend. In addition, there are limitations for when
goto
can be used. For example, it is not permitted to
jump past a statement that initializes a local object having a
destructor.
Variables representing size or length are typical candidates for
unsigned
declarations. By following this recommendation
some unpleasant errors can be avoided.
It is best to use inclusive lower and exclusive upper limits.
Instead of saying that x is in the interval x>=23
and
x<=42
, use the limits x>=23
and
x<43
. The following important claims then apply:
By being consistent in this regard, many difficult errors will be avoided.
If the code which follows a case label is not terminated by
break
, the execution continues after the next case label.
This means that poorly tested code can be erroneous and still seem to
work.
continue
can be used to exit from loops. However,
code may be more comprehensible by using an else
clause
instead.
C++ has a very loose and, simultaneously, very free way of determining if an expression is true or false. If an expression is evaluated as 0, it is false; otherwise, it is considered to be true.
We do not recommend logical tests such as if(pointer)
if pointer
is a variable of pointer-type. The only
reason is readablity; many programmers find it difficult to read such
code.
Consider the scope within which an iteration variable is visible.
A variable that is declared within a for
statement is
currently only visible in the nearest enclosing block. The
standardization committee for C++ is however discussing a language
modification regarding this point. No decision has yet been made.
Still, this problem is avoided if the control structure is
encapsulated in a compound statement.
When several case
labels are followed by the same
block of code, only one break
statement is needed. Also,
other statements than break
may be used to exit a
switch
statement, such as return
.
No exceptions.
For extremely time-critical applications or for fault handling,
goto
may be permitted. Every such usage must be
carefully motivated.
Problem using unsigned
loop variables
for( unsigned int i = 3; i >= 0; --i ) { // This loop will never terminate, since i cycles through: // 3, 2, 1, 0,<PHONE_NUMBER>,<PHONE_NUMBER>, etc ... on a SparcStation // Note that this example does not follow the rules: i >= 0 // in the for statement. See next example ! }
Visibility of variable declared in for
loop
for ( int index = 0; index < 10; index++ ) { cout << index; } int index = 3; // ERROR, THIS IS AN ILLEGAL RE-DECLARATION OF index // BECAUSE index IS DECLARED IN BLOCK-SCOPE.
Dangerous switch/case statement
switch ( tag ) { case A: { // Do something // Next statement is a call to foo() inside next case } case B: { foo(); // Do something else break; // Now we leave the switch-statement } default: { // If no match in above cases, this is executed exit( 1 ); } }
Good and bad ways of setting limits for loop variables
int a[10]; int ten = 10; int nine = 9; // Good way to do it: for( int i = 0; i < ten; i++ ) // Loop runs 10-0=10 times { a[i] = 0; } // Bad way to do it: for( int j = 0; j <= nine; j++ ) // Loop runs 10 times, but 9-0=9 !!! { a[j] = 0; }
Using break
to exit a loop, no flags are needed.
do // This way: { if ( Something ) { // Do something break; } } while( someCondition ); int endFlag = 0; // Is better than this: do { if ( /* Something */ ) { // Do something endFlag = 1; } } while( someCondition && !endFlag );
By using an extra `else
' clause, continue
is avoided and the code can be comprehended.
while( /* Something */ ) // This way is more clear { if( /* Something */ ) { // Do something } else { // Do something else } } while( /* Something */ ) // Than using continue { if( /* Something */ ) { // Do something continue; // No ! } // Do something else }
Use parentheses to clarify the order of evaluation for operators in expressions.
There are a number of common pitfalls having to do with the order of evaluation for operators in an expression. Binary operators in C++ have associativity (either leftwards or rightwards) and precedence. If an operator has leftwards associativity and occurs on both sides of a variable in an expression, then the variable belongs to the same part of the expression as the operator on its left side.
In doubtful cases, parentheses always are to be used to clarify the order of evaluation.
Another common mistake is to confuse the assignment operator and the equality operator. Since the assignment operator returns a value, it is entirely permitted to have an assignment statement instead of a comparison expression. This, however, most often leads straight to an error.
C++ allows the overloading of operators, something which can easily
become confusing. For example, the operators <<
(shift left) and >>
(shift right) are often used
for input and output. Since these were originally bit operations, it
is necessary that they have higher priority than relational operators.
This means that parentheses must be used when outputting the values of
logical expressions.
Problem with the order of evaluation
// Interpreted as ( a<b )<c, not ( a<b ) && ( b<ac ) if ( a<b<c ) { // ... } // Interpreted as a & ( b<8 ), not ( a & b )<8 if ( a & b<8 ) { // ... }
When parentheses are recommended
int i = a >= b && c<d && e + f < = g + h; // No! int j = ( a >= b ) && ( c<d ) && (( e + f ) < = ( g + h )); // Better
Do not use malloc
, realloc
or
free
.
Always provide empty brackets ("[]
") for
delete
when deallocating arrays.
Avoid global data if at all possible.
Do not allocate memory and expect that someone else will deallocate it later.
Always assign a new value to a pointer that points to deallocated memory.
In C++ data can be allocated statically, dynamically on the stack, or dynamically on the heap. There are three categories of static data: global data, global class data, and static data local to a function.
In C malloc
, realloc
and
free
are used to allocate memory dynamically on the heap.
This may lead to conflicts with the use of the new
and
delete
operators in C++.
It is dangerous to:
delete
for a pointer obtained via
malloc
/realloc
,malloc
/realloc
for objects
having constructors,free
for anything allocated using
new
.Thus, avoid whenever possible the use of malloc
,
realloc
and free
.
If an array a
having a type T
is
allocated, it is important to invoke delete
in the
correct way. Only writing delete a;
will result in the
destructor being invoked only for the first object of type
T
. By writing delete [m] a;
where
m
is an integer which is greater than the number of
objects allocated earlier, the destructor for T
will be
invoked for memory that does not represent objects of type
T
. The easiest way to do this correctly is to write
delete [] a;
since the destructor will then be invoked
only for those objects which have been allocated earlier.
Static data can cause several problems. In an environment where parallel threads execute simultaneously, they can make the behaviour of code unpredictable, since functions having static data are not reentrant.
One difference between ANSI-C and C++ is in how constants are
declared. If a variable is declared as a constant in ANSI-C, it has
the storage class extern
(global). In C++, however, it
normally has the storage class static
(local). The
latter means that a new instance of the constant object is created
each time a file includes the file which contains the declaration of
the object, unless the variable is explicitly declared
extern
in the include file.
An extern declaration in C++ does not mean that the variable is initialized; there must be a definition for this in a definition file. Static constants that are defined within a class are always external and must always be defined separately.
It may, at times, be tempting to allocate memory for an object
using new
, expecting someone else to deallocate the
memory. For instance, a function can allocate memory for an object
which is then returned to the user as the return value for the
function. There is no guarantee that the user will remember to
deallocate the memory and the interface with the function then becomes
considerably more complex.
Pointers that point to deallocated memory should either be set to 0 or be given a new value to prevent access to the released memory. This can be a very difficult problem to solve when there are several pointers which point to the same memory, since C++ has no garbage collection.
No exceptions.
No exceptions.
Right and wrong ways to invoke delete
for arrays with
destructors
int n = 7; T* myT = new T[n]; // T is a type with defined constructors and destructors // ... delete myT; // No! Destructor only called for first object in array a delete [10] myT; // No! Destructor called on memory out of bounds in array a delete [] myT; // OK, and always safe!
Dangerous memory management
String myFunc( const char* myArgument ) { String* temp = new String( myArgument ); return *temp; // temp is never deallocated and the user of myFunc cannot deallocate // because a temporary copy of that instance is returned. }
Make sure that fault handling is done so that the transfer to exception handling (when this is available in C++) may be easily made.
Check the fault codes which may be received from library functions even if these functions seem foolproof.
In November 1990, the ANSI C++ committee accepted a proposal for
exception handling which is described in chapter 15 of ref. 1. When
designing fault handling in code, it is appropriate to consider being
able to make a smooth transfer to exception handling. For example,
instead of using ordinary fault codes, which may necessitate a lot of
re-programming when exception handling is available, a call can be
made to a function void fault(const char*)
which sends a
fault message (somewhere) and then terminates execution in some
way.
System functions (those which are specific to UNIX) ought to be used with care if the code is to be portable. If such functions are used, the possible fault codes that may be received should be carefully checked.
Two important characteristics of a robust system are that all faults are reported and, if the fault is so serious that continued execution is not possible, the process is terminated. In this way, the propagation of faults through the system is avoided. It is better to have a process crash, than to spread erroneous information to other processes. In achieving this goal, it is important to always test fault codes from library functions. The opening or closing of files may fail, allocation of data may fail, etc. One test too many is better than one test too few. Our own functions should preferably not return fault codes, but should instead take advantage of exception handling.
Future exception handling in C++
// The top function where we catch exceptions thrown in called functions int f() { // We suspect that something can go wrong when function g() is called. // Therefore, we enclose the call in a try block. try { return g(); // This is the try block } // If any exceptions, having a given type, were thrown when g() // was executing, they are caught in these two catch blocks. catch ( int x ) // catches int { cerr << "Number " << x << " happened !" << endl; return x; } catch ( char* x ) // catches char* { // Respond in some other way } // Anything else that is thrown, is thrown up to the function that calls f() } // This function has no try or catch block. When the exception is thrown // in function h(), it is thrown up to the function f(). int g() { return h(); } extern int somethingIsVeryWrongAndICannotHandleThisAnyMore(); int h() { // Here we find out that something went wrong, and throw an exception if (somethingIsVeryWrongAndICannotHandleThisAnyMore()) { // In this case, we throw an int as exception, but almost any object // can be thrown. See Errata for "The Annotated C++ Reference Manual" // section 15.7. throw 2; } // Keep on trucking if all is OK }
Avoid the direct use of pre-defined data types in declarations.
An excellent way of transforming your world to a "vale of tears" is
to directly use the pre-defined data types in declarations. If it is
later necessary, due to portability problems, to change the return
type of a function, it may be necessary to make change at a large
number of places in the code. One way to avoid this is to declare a
new type name using classes or typedefs to represent the types of
variables used. In this way, changes can be more easily made. This
may be used to give data a physical unit, such as kilogram or meter.
Such code is more easily reviewed. (For example, when the code is
functioning poorly, it may be noticed that a variable representing
meters has been assigned to a variable representing kilograms.) It
should be noted that a typedef
does not create a new
type, only an alternative name for a type. This means that if you
have declared typedef int Error
, a variable of the type
Error
may be used anywhere where an int
may
be used.
See also chapter 12, Rec. 49!
Type declarations using typedef
// Instead of: long int time; short int mouseX; char* menuName; // Use (for example): typedef long int TimeStamp; typedef short int Coordinate; class String { /* ... */ }; // and: TimeStamp time; Coordinate mouseX; String menuName;
Do not assume that an int
and a long
have
the same size.
Do not assume that an int
is 32 bits long (it may be
only 16 bits long).
Do not assume that a char
is signed or unsigned.
Always set char
to unsigned if 8-bit ASCII is used.
In the definition of the C++ language, it has not yet been decided
if a char
is signed or unsigned. This decision has
instead been left to each compiler manufacturer. If this is forgotten
and this characteristic is exploited in one way or another, some
difficult bugs may appear in the program when another compiler is
used.
If 8-bit ASCII is used (as is quite likely in the future) and
comparisons are made of two characters, it is important that
unsigned char
is used.
Be careful not to make type conversions from a "shorter" type to a "longer" one.
Do not assume that pointers and integers have the same size.
Use explicit type conversions for arithmetic using signed and unsigned values.
A processor architecture often forbids data of a given size to be
allocated at an arbitrary address. For example, a word must begin on
an "even" address for MC680x0. If there is a pointer to a
char
which is located at an "odd" address, a type
conversion from this char
pointer to an int
pointer will cause the program to crash when the int
pointer is used, since this violates the processor's rules for
alignment of data.
Do not assume that you know how an instance of a data type is represented in memory.
Do not assume that long
s, float
s, double
s or long double
s may begin at arbitrary addresses.
The representation of data types in memory is highly machine-dependent. By allocating data members to certain addresses, a processor may execute code more efficiently. Because of this, the data structure that represents a class will sometime include holes and be stored differently in different process architectures. Code which depends on a specific representation is, of course, not portable.
See 18.3 for explanation of Port. Rec. 10.
Do not depend on underflow or overflow functioning in any special way.
Do not assume that the operands in an expression are evaluated in a definite order.
Do not assume that you know how the invocation mechanism for a function is implemented.
Do not assume that an object is initialized in any special order in constructors.
Do not assume that static objects are initialized in any special order.
If a value is modified twice in the same expression, the result of the expression is undefined except when the order of evaluation is guaranteed for the operators that are used.
The order of initialization for static objects may present problems. A static object may not be used in a constructor, if it is not initialized until after the constructor is run. At present, the order of initialization for static objects, which are defined in different compilation units, is not defined. This can lead to errors that are difficult to locate (see Example 69). There are special techniques for avoiding this. See Example 29!
Do not depend on the order of initialization in constructors.
#include <iostream.h> class X { public: X(int y); private: int i; int j; }; inline X::X(int y) : j(y), i(j) // No! j may not be initialized before i !! { cout << "i:" << i << " & " << "j:" << j << endl; } main() { X x(7); // Rather unexpected output: i:0 & j:7 }
Initialization of static objects
// Foo.hh #include <iostream.h> #include <string.h> static unsigned int const Size = 1024; class Foo { public: Foo( char* cp ); // Constructor // ... private: char buffer[Size]; static unsigned counter; // Number of constructed Foo:s }; extern Foo foo_1; extern Foo foo_2; // Foo1.cc #include "Foo.hh" unsigned Foo::counter = 0; Foo foo_1 = "one"; //Foo2.cc #include "Foo.hh" Foo foo_2 = "two"; Foo::Foo( char* cp ) // Irrational constructor { strncpy( buffer, cp, sizeof(buffer) ); foos[counter] = this; switch ( counter++ ) { case 0: case 1: cout << ::foo_1.buffer << "," << ::foo_2.buffer << endl; break; default: cout << "Hello, world" << endl; } } // If a program using Foo.hh is linked with Foo1.o and Foo2.o, either // ,two or one, is written on standard output depending on // one,two one,two the order of the files given to the linker.
Do not write code which is dependent on the lifetime of a temporary object.
Temporary objects are often created in C++, such as when functions return a value. Difficult errors may arise when there are pointers in temporary objects. Since the language does not define the life expectancy of temporary objects, it is never certain that pointers to them are valid when they are used.
One way of avoiding this problem is to make sure that temporary objects are not created. This method, however, is limited by the expressive power of the language and is not generally recommended.
The C++ standard may someday provide an solution to this problem. In any case, it is a subject for lively discussions in the standardization committee.
Difficult error in a string class which lacks output operator
class String { public: operator const char*() const; // Conversion operator to const char* friend String operator+( const String& left, const String& right ); // ... }; String a = "This may go to "; String b = "h***!"; // The addition of a and b generates a new temporary String object. // After it is converted to a char* by the conversion operator, it is // no longer needed and may be deallocated. This means that characters // which are already deallocated are printed to cout -> DANGEROUS!! cout << a + b;
Avoid using shift operations instead of arithmetic operations.
Avoid pointer arithmetic.
Pointer arithmetic can be portable. The operators ==
and !=
are defined for all pointers of the same type,
while the use of the operators <
, >
,
<=
, >=
are portable only if they are
used between pointers which point into the same array.
lint++
*
) or references (&
) shall not be
included as include files.#include
directives._
' or `__
')._
')._
').public
, protected
, and
private
sections of a class are to be declared in that
order (the public
section is declared before the
protected
section which is declared before the
private
section).public
or protected
member data in a class.const
.const
member
functions.new
" to allocate instances
managed by the class, must define a copy constructor.new
" to allocate instances
managed by the class, must define an assignment operator.const
reference or pointer to member data.const
reference or pointer to data outside an
object, unless the object shares the data with other objects.#define
to
obtain more efficient code; instead, use inline functions.const
or
enum
; never using #define
.NULL
or assign
NULL
to a pointer; use 0
instead.const
to a non-const
.case
label must always be
terminated by a break
statement.switch
statement must always contain a
default
branch which handles unexpected cases.goto
.malloc
, realloc
or
free
.[]
") for
delete
when deallocating arrays.+w
flag set to eliminate as many warnings as
possible.//
for comments.#include "filename.hh"
for user-prepared include files.#include
<filename.hh>
for include files from
libraries.what
can be
used to obtain information on the file revision.{}
") which enclose a block are to be placed
in the same column, on separate lines directly before and after the
block.if
, else
,
while
, for
and do
should be
followed by a block, even if it is an empty block.*
' and the address-of
operator `&
' should be directly connected with the
type names in declarations and definitions..
' or
`->
', nor between unary operators and operands.inline
.inline
.inline
.const
reference to the assigning object.==
and
!=
), it is appropriate to define both.protected
access functions.const &
) instead of
call-by-value, unless using a pre-defined data type or a
pointer.typedef
to simplify program syntax when
declaring function pointers.for
,
while
or do
-while
) should
depend on the specific use of the loop.unsigned
for variables which cannot
reasonably have negative values.continue
.break
to exit a loop if this avoids the use of
flags.if(test)
or if(!test)
when
test
is a pointer.int
and a long
have the same size.int
is 32 bits long (it may be
only 16 bits long).char
is signed or
unsigned.char
to unsigned if 8-bit ASCII is
used.long
s, float
s,
double
s or long double
s may begin at
arbitrary addresses.Marko Mäkelä (msmakela "at" tcs dot hut dot fi) overhauled the document in 2001 to make it valid HTML and added more hyperlinks.
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