Preprocessor

The preprocessor is executed at translation phase 4, before the compilation. The result of preprocessing is a single file which is then passed to the actual compiler.

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 where the beginning token is not a standard-defined directive name, in this case the directive is conditionally-supported with implementation-defined semantics (e.g. a common extension is the directive #warning which emits a user-defined message during compilation) (until C++23) , or
  • nothing, in this case the directive has no effect.
• a line break.

The module and import directives are also preprocessing directives. (since C++20)

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++23) ).
• cause an error or warning (since C++17) (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 an extension.
• 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.

CWG 2001 (C++98)

Link	https://cplusplus.github.io/CWG/issues/2001.html
Applied to	C++98
Behavior as published	the behavior of using non-standard-defined directives was not clear
Correct behavior	made conditionally-supported

See also

C++ documentation for Predefined Macro Symbols

C++ documentation for Macro Symbol Index

C documentation for preprocessor