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.
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 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