# Preprocessor

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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 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 # 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

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