Preprocessor
From cppreference.com
< cpp
C++
Feature test macros (C++20)
Concepts library (C++20)
Metaprogramming library (C++11)
Ranges library (C++20)
Filesystem library (C++17)
Concurrency support library (C++11)
Execution control library (C++26)
C++ language
Flow control Conditional execution statements Iteration statements (loops) Jump statements Functions Function declaration Lambda function expression Dynamic exception specifications (until C++17*) Exceptions Namespaces Types Specifiers Storage duration specifiers Initialization Alternative representations Literals Boolean - Integer - Floating-point Character - String - nullptr (C++11) User-defined (C++11) Utilities Attributes (C++11) Types Type alias declaration (C++11) Casts Memory allocation Classes Class-specific function properties Special member functions Templates Miscellaneous
General topics
Preprocessor
inline specifier noexcept specifier (C++11)typedef declaration Preprocessor
(C++23)(C++23)
(C++17)
(C++23)
(C++11)
(C++26)
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.
[edit] 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
[edit] 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).
[edit] 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 |
[edit] See also
C++ documentation for Predefined Macro Symbols
C++ documentation for Macro Symbol Index
C documentation for preprocessor