 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 # 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), 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 of 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 
  

    
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