Preprocessor directives
#
). These lines are not program statements but directives for the preprocessor. The preprocessor examines the code before actual compilation of code begins and resolves all these directives before any code is actually generated by regular statements.
These preprocessor directives extend only across a single line of code. As soon as a newline character is found, the preprocessor directive is ends. No semicolon (;
) is expected at the end of a preprocessor directive. The only way a preprocessor directive can extend through more than one line is by preceding the newline character at the end of the line by a backslash (\
).
macro definitions (#define, #undef)
To define preprocessor macros we can use #define
. Its syntax is:
#define identifier replacement
When the preprocessor encounters this directive, it replaces any occurrence of identifier
in the rest of the code by replacement
. This replacement
can be an expression, a statement, a block or simply anything. The preprocessor does not understand C++ proper, it simply replaces any occurrence of identifier
by replacement
.
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After the preprocessor has replaced TABLE_SIZE
, the code becomes equivalent to:
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#define
can work also with parameters to define function macros:
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This would replace any occurrence of getmax
followed by two arguments by the replacement expression, but also replacing each argument by its identifier, exactly as you would expect if it was a function:
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5 7 |
Defined macros are not affected by block structure. A macro lasts until it is undefined with the #undef
preprocessor directive:
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This would generate the same code as:
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Function macro definitions accept two special operators (#
and ##
) in the replacement sequence:
If the operator #
is used before a parameter is used in the replacement sequence, that parameter is replaced by a string literal (as if it were enclosed between double quotes)
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This would be translated into:
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The operator ##
concatenates two arguments leaving no blank spaces between them:
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This would also be translated into:
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Because preprocessor replacements happen before any C++ syntax check, macro definitions can be a tricky feature. But, be careful: code that relies heavily on complicated macros become less readable, since the syntax expected is on many occasions different from the normal expressions programmers expect in C++.
Conditional inclusions (#ifdef, #ifndef, #if, #endif, #else and #elif)
These directives allow to include or discard part of the code of a program if a certain condition is met.
#ifdef
allows a section of a program to be compiled only if the macro that is specified as the parameter has been defined, no matter which its value is. For example:
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In this case, the line of code int table[TABLE_SIZE];
is only compiled if TABLE_SIZE
was previously defined with #define
, independently of its value. If it was not defined, that line will not be included in the program compilation.
#ifndef
serves for the exact opposite: the code between #ifndef
and #endif
directives is only compiled if the specified identifier has not been previously defined. For example:
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In this case, if when arriving at this piece of code, the TABLE_SIZE
macro has not been defined yet, it would be defined to a value of 100. If it already existed it would keep its previous value since the #define
directive would not be executed.
The #if
, #else
and #elif
(i.e., “else if”) directives serve to specify some condition to be met in order for the portion of code they surround to be compiled. The condition that follows #if
or #elif
can only evaluate constant expressions, including macro expressions. For example:
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Notice how the entire structure of #if
, #elif
and #else
chained directives ends with #endif
.
The behavior of #ifdef
and #ifndef
can also be achieved by using the special operators defined
and !defined
respectively in any #if
or #elif
directive:
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Line control (#line)
When we compile a program and some error happens during the compiling process, the compiler shows an error message with references to the name of the file where the error happened and a line number, so it is easier to find the code generating the error.
The #line
directive allows us to control both things, the line numbers within the code files as well as the file name that we want that appears when an error takes place. Its format is:
#line number "filename"
Where number
is the new line number that will be assigned to the next code line. The line numbers of successive lines will be increased one by one from this point on.
"filename"
is an optional parameter that allows to redefine the file name that will be shown. For example:
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This code will generate an error that will be shown as error in file "assigning variable"
, line 20.
Error directive (#error)
This directive aborts the compilation process when it is found, generating a compilation error that can be specified as its parameter:
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This example aborts the compilation process if the macro name __cplusplus
is not defined (this macro name is defined by default in all C++ compilers).
This directive has been used assiduously in other sections of this tutorial. When the preprocessor finds an In the first case, a header is specified between angle-brackets The syntax used in the second Source file inclusion (#include)
#include
directive it replaces it by the entire content of the specified header or file. There are two ways to use #include
:
1
2
#include <header>
#include "file"
<>
. This is used to include headers provided by the implementation, such as the headers that compose the standard library (iostream
, string
,…). Whether the headers are actually files or exist in some other form is implementation-defined, but in any case they shall be properly included with this directive.#include
uses quotes, and includes a file. The file is searched for in an implementation-defined manner, which generally includes the current path. In the case that the file is not found, the compiler interprets the directive as a header inclusion, just as if the quotes (""
) were replaced by angle-brackets (<>
).
Pragma directive (#pragma)
This directive is used to specify diverse options to the compiler. These options are specific for the platform and the compiler you use. Consult the manual or the reference of your compiler for more information on the possible parameters that you can define with #pragma
.
If the compiler does not support a specific argument for #pragma
, it is ignored – no syntax error is generated.
Predefined macro names
The following macro names are always defined (they all begin and end with two underscore characters, _
):
macro | value |
---|---|
__LINE__ |
Integer value representing the current line in the source code file being compiled. |
__FILE__ |
A string literal containing the presumed name of the source file being compiled. |
__DATE__ |
A string literal in the form “Mmm dd yyyy” containing the date in which the compilation process began. |
__TIME__ |
A string literal in the form “hh:mm:ss” containing the time at which the compilation process began. |
__cplusplus |
An integer value. All C++ compilers have this constant defined to some value. Its value depends on the version of the standard supported by the compiler:
Non conforming compilers define this constant as some value at most five digits long. Note that many compilers are not fully conforming and thus will have this constant defined as neither of the values above. |
__STD_HOSTED__ |
1 if the implementation is a hosted implementation (with all standard headers available)0 otherwise. |
The following macros are optionally defined, generally depending on whether a feature is available:
macro | value |
---|---|
__STDC__ |
In C: if defined to 1 , the implementation conforms to the C standard.In C++: Implementation defined. |
__STDC_VERSION__ |
In C:
In C++: Implementation defined. |
__STDC_MB_MIGHT_NEQ_WC__ |
1 if multibyte encoding might give a character a different value in character literals |
__STDC_ISO_10646__ |
A value in the form yyyymmL , specifying the date of the Unicode standard followed by the encoding of wchar_t characters |
__STDCPP_STRICT_POINTER_SAFETY__ |
1 if the implementation has strict pointer safety (see get_pointer_safety ) |
__STDCPP_THREADS__ |
1 if the program can have more than one thread |
Particular implementations may define additional constants.
For example:
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This is the line number 7 of file /home/jay/stdmacronames.cpp. Its compilation began Nov 1 2005 at 10:12:29. The compiler gives a __cplusplus value of 1 |
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