Namespaces
Variants
Views
Actions

Preprocessor

From cppreference.com
< cpp
 
 
C++ language
General topics
Preprocessor
Comments
Flow control
Conditional execution statements
if
Iteration statements (loops)
for
range-for (C++11)
Jump statements
Functions
Function declaration
Lambda function expression
inline specifier
Dynamic exception specifications (until C++20)
noexcept specifier (C++11)
Exceptions
Namespaces
Types
Specifiers
decltype (C++11)
auto (C++11)
alignas (C++11)
Storage duration specifiers
Initialization
Expressions
Alternative representations
Literals
Boolean - Integer - Floating-point
Character - String - nullptr (C++11)
User-defined (C++11)
Utilities
Attributes (C++11)
Types
typedef declaration
Type alias declaration (C++11)
Casts
Implicit conversions - Explicit conversions
static_cast - dynamic_cast
const_cast - reinterpret_cast
Memory allocation
Classes
Class-specific function properties
explicit (C++11)
static
Special member functions
Templates
Miscellaneous
 
 

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.

Contents

[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), 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 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)

[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