Conditional inclusion

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

 

C++ language

General topics
Preprocessor Comments Keywords	Escape sequences History of C++
Flow control
Conditional execution statements
if statement	switch statement
Iteration statements
for loop range-for loop (C++11)	while loop do-while loop
Jump statements
continue statement break statement	goto statement return statement
Functions
function declaration
lambda function declaration
function template
inline specifier
exception specifications (deprecated)
noexcept specifier (C++11)
Exceptions
throw expression	try-catch block
Namespaces
namespace declaration	namespace aliases
Types
fundamental types compound types enumeration types	union types function types
decltype specifier (C++11)
Specifiers
cv specifiers
storage duration specifiers
constexpr specifier (C++11)
auto specifier (C++11)
alignas specifier (C++11)
Initialization
default initialization value initialization zero initialization copy initialization direct initialization	aggregate initialization list initialization (C++11) constant initialization reference initialization

Literals
integer literals character literals floating-point literals string literals	boolean literals nullptr (C++11) user-defined (C++11)
Expressions
value categories order of evaluation	operators operator precedence
alternative representations
Utilities
Types
typedef declaration
type alias declaration (C++11)
attributes (C++11)
Casts
implicit conversions
const_cast conversion
static_cast conversion
dynamic_cast conversion
reinterpret_cast conversion
C-style and functional cast
Memory allocation
new expression	delete expression
Classes
class declaration this pointer access specifiers	friend specifier initializer lists
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

 

Preprocessor

#if#ifdef#ifndef#else#elif#endif

#define#undef#,## operators

#include

#error

#pragma

#line

 

The preprocessor supports conditional compilation of parts of source file. This behavior is controlled by #if, #else, #elif, #ifdef, #ifndef and #endif directives.

Syntax

#if expression
#ifdef expression
#ifndef expression
#elif expression
#else
#endif

Explanation

The conditional preprocessing block starts with #if, #ifdef or #ifndef directive, then optionally includes any number of #elif directives, then optionally includes at most one #else directive and is terminated with #endif directive. Any inner conditional preprocessing blocks are processed separately.

Each of #if, #elif, #else, #ifdef and #ifndef directives control code block until first #elif, #else, #endif directive not belonging to any inner conditional preprocessing blocks.

#if, #ifdef and #ifndef directives test the specified condition (see below) and if it evaluates to true, compiles the controlled code block. In that case subsequent #else and #elif directives are ignored. Otherwise, if the specified condition evaluates false, the controlled code block is skipped and the subsequent #else or #elif directive (if any) is processed. In the former case, the code block controlled by the #else directive is unconditionally compiled. In the latter case, the #elif directive acts as if it was #if directive: checks for condition, compiles or skips the controlled code block based on the result, and in the latter case processes subsequent #elif and #else directives. The conditional preprocessing block is terminated by #endif directive.

Condition evaluation

#if, #elif

The expression is a constant expression, using only literals and identifiers, defined using #define directive. Any identifier, which is not literal, non defined using #define directive, evaluates to 0.

The expression may contain unary operators in form defined identifier or defined (identifier) which return 1 if the identifier was defined using #define directive and 0 otherwise. If the expression evaluates to nonzero value, the controlled code block is included and skipped otherwise. If any used identifier is not a constant, it is replaced with ​0​.

#ifdef, #ifndef

Checks if the identifier was defined using #define directive.

#ifdef identifier is essentially equivalent to #if defined( identifier).

#ifndef identifier is essentially equivalent to #if !defined( identifier).

Example

Run this code

#define ABCD 2
#include <iostream>
 
int main()
{
 
#ifdef ABCD
    std::cout << "1: yes\n";
#else
    std::cout << "1: no\n";
#endif
 
#ifndef ABCD
    std::cout << "2: no1\n";
#elif ABCD == 2
    std::cout << "2: yes\n";
#else
    std::cout << "2: no2\n";
#endif
 
#if !defined(DCBA) && (ABCD < 2*4-3)
    std::cout << "3: yes\n";
#endif
}

Output:

1: yes
2: yes
3: yes