Preprocessor

The preprocessor is executed at translation phase 4

Directives

The preprocessing directives control the behavior of the preprocessor. Each directive occupies one line and has the following format:

• the # character.
• a sequence of:

• a standard-defined directive name (listed below) followed by the corresponding arguments, or
• one or more preprocessing tokens (e.g. a common non-standard extension is the directive #warning which emits a user-defined message during compilation) (until C++23)
• nothing, in this case the directive has no effect.

• a line break.

Preprocessing directives must not come from macro expansion.

#define EMPTY
EMPTY   #   include <file.h> // not a preprocessing directive

Capabilities

The preprocessor has the source file translation capabilities:

• conditionally compile parts of source file (controlled by directive #if, #ifdef, #ifndef, #else, #elif , #elifdef, #elifndef (since C++23) , and #endif
• replace text macros while possibly concatenating or quoting identifiers (controlled by directives #define and #undef, and operators # and ##
• include other files (controlled by directive #include and checked with __has_include(since C++17)
• cause an error or warning(since C++23) (controlled by directive #error or #warning respectively (since C++23)

The following aspects of the preprocessor can be controlled:

• implementation-defined behavior (controlled by directive #pragma and operator _Pragma (since C++11) ). In addition, some compilers support (to varying degrees) the operator __pragma as a non-standard
• file name and line information available to the preprocessor (controlled by directive #line

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also