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.


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 non-standard 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


The preprocessor has the source file translation capabilities:

  • conditionally compile of 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 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.

DR Applied to Behavior as published Correct behavior
CWG 2001 C++98 the behavior of using non-standard-defined directives was not clear made conditionally-supported

See also

C++ documentation for Predefined Macro Symbols
C++ documentation for Macro Symbol Index
C documentation for preprocessor

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