Conditional inclusion

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

Syntax

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.

If the expression contains unary operators in form defined identifier or defined (identifier), it is evaluated first. The result is 1 if the identifier was defined as a macro name or the identifier is __has_include (since C++17), otherwise the result is ​0​.

After macro expansion and evaluation of the defined operator, any identifier which is not a boolean literal, nor currently defined as a macro name, are replaced with the number ​0​.

If the expression evaluates to nonzero value, the controlled code block is included and skipped otherwise.

#ifdef, #ifndef

Checks if the identifier was defined as a macro name.

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

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

Example

#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
}

1: yes
2: yes
3: yes

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also