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Conditional inclusion

From cppreference.com

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

Contents

[edit] Syntax

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

[edit] 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 the #endif directive. Any inner conditional preprocessing blocks are processed separately.

Each of #if, #elif, #else, #ifdef and #ifndef directives control a code block until the 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 were a #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 the #endif directive.

[edit] Conditional evaluation

[edit] #if, #elif

The expression is a constant expression, using only constants 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.

In context of a preprocessor directive, a __has_c_attribute expression detects whether a given attribute token is supported and its supported version. See Attribute testing.

(since C23)

Note: Until DR 412, #if cond1 ... #elif cond2 is different from #if cond1 ... #else followed by #if cond3 because if cond1 is true, the second #if is skipped and cond3 does not need to be well-formed, while #elif's cond2 must be a valid expression. As of DR 412, #elif that leads the skipped code block is also skipped.

[edit] #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.

[edit] Example

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

Output:

1: yes
2: yes
3: yes

[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
DR 412 C89 failed #elif's expression was required to be valid failed #elif is skipped

[edit] References

  • C17 standard (ISO/IEC 9899:2018):
  • 6.10.1 Conditional inclusion (p: 118-119)
  • C11 standard (ISO/IEC 9899:2011):
  • 6.10.1 Conditional inclusion (p: 162-164)
  • C99 standard (ISO/IEC 9899:1999):
  • 6.10.1 Conditional inclusion (p: 147-149)
  • C89/C90 standard (ISO/IEC 9899:1990):
  • 3.8.1 Conditional inclusion

[edit] See also

C++ documentation for Conditional inclusion