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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 #.
  • 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

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 #, ##). 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.

DRApplied toBehavior as publishedCorrect behavior
CWG 2001(C++98)the behavior of using non-standard-defined directives was not clearmade conditionally-supported

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 #.
  • 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

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 #, ##). 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.

DRApplied toBehavior as publishedCorrect behavior
CWG 2001(C++98)the behavior of using non-standard-defined directives was not clearmade conditionally-supported