조건부 포함

선행처리기는 여러 소스 파일들간의 조건부 결합을 지원합니다. 관련 구문은 아래와 같습니다.

[편집] 구문

[편집] 설명

조건부 선행처리는 #if, #ifdef 또는 #ifndef 명령어로 시작되고, 이후로 선택적으로 여러 개의 #elif 명령어와 적어도 한 개의 #else 명령어를 작성한 후 #endif 명령어로 종료됩니다. 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.

[편집] Condition evaluation

[편집] #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 are replaced with the number ​0​.

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

[편집] #ifdef, #ifndef

식별자가 이미 매크로로 정의되었는지 확인합니다.

#ifdef identifier은 #if defined identifier와 같다..

#ifndef identifier은 #if !defined identifier와 같다..

[편집] 예시

#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

[편집] Defect reports

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

[편집] See also