Preprocessor

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

C++ language

General topics
Preprocessor Comments	Keywords Escape sequences
Flow control
Conditional execution statements
if	switch
Iteration statements (loops)
for range-for (C++11)	while do-while
Jump statements
continue - break	goto - return
Functions
Function declaration
Lambda function expression
inline specifier
Dynamic exception specifications (until C++17*)
noexcept specifier (C++11)
Exceptions
throw-expression try block	handler
Namespaces
Namespace declaration	Namespace aliases
Types
Fundamental types Enumeration types Function types	Class/struct types Union types
Specifiers
const/volatile decltype (C++11) auto (C++11)	constexpr (C++11) consteval (C++20) constinit (C++20)
Storage duration specifiers
Initialization
Default-initialization Value-initialization Zero-initialization Copy-initialization Direct-initialization	Aggregate initialization List-initialization (C++11)   Constant initialization Reference initialization

Expressions
Value categories Order of evaluation	Operators Operator precedence
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 static_cast const_cast	Explicit conversions dynamic_cast reinterpret_cast
Memory allocation
new expression	delete expression
Classes
Class declaration Constructors this pointer	Access specifiers friend specifier
Class-specific function properties
Virtual function override specifier (C++11)   final specifier (C++11)	explicit (C++11) static
Special member functions
Default constructor Copy constructor Move constructor (C++11)	Copy assignment Move assignment (C++11) Destructor
Templates
Class template Function template	Template specialization Parameter packs (C++11)
Miscellaneous
Inline assembly	History of C++

 

Preprocessor

#if#ifdef#ifndef#else#elif#elifdef#elifndef#endif (C++23)(C++23)

#define#undef#,## operators

#include__has_include (C++17)

#error#warning (C++23)

#pragma_Pragma (C++11)

#line

 

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

C++ documentation for Predefined Macro Symbols

C++ documentation for Macro Symbol Index

C documentation for preprocessor