Conditional inclusion
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 |
(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
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
|