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:

• # character
• preprocessing instruction (one of define, undef, include, if, ifdef, ifndef, else, elif, endif, line, error, pragma) ^[1]
• arguments (depends on the instruction)
• line break

The null directive (# followed by a line break) is allowed and has no effect.

Capabilities

The preprocessor has the source file translation capabilities:

• conditionally compile of parts of source file (controlled by directive #if, #ifdef, #ifndef, #else, #elif 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 (controlled by directive #error)

The following aspects of the preprocessor can be controlled:

• implementation defined behavior (controlled by directive #pragma and operator _Pragma (since C++11))
• file name and line information available to the preprocessor (controlled by directive #line)

Footnotes

1. These are the directives defined by the standard. The standard does not define behavior for other directives: they might be ignored, have some useful meaning, or cause a compile-time error. Even if otherwise ignored, they are removed from the source code when the preprocessor is done. A common non-standard extension is the directive #warning which emits a user-defined message during compilation.

See also