Conditional inclusion
The preprocessor supports conditional compilation of parts of source file. This behavior is controlled by #if, #else, #elif, #ifdef, #ifndef, #elifdef, #elifndef(since C++23), and #endif directives.
Contents |
[edit]Syntax
#ifexpression | |||||||||
#ifdefidentifier | |||||||||
#ifndefidentifier | |||||||||
#elifexpression | |||||||||
#elifdefidentifier | (since C++23) | ||||||||
#elifndefidentifier | (since C++23) | ||||||||
#else | |||||||||
#endif | |||||||||
[edit]Explanation
The conditional preprocessing block starts with #if, #ifdef or #ifndef directive, then optionally includes any number of #elif, #elifdef, or #elifndef(since C++23) 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, #ifdef, #ifndef, #elif, #elifdef, #elifndef(since C++23), and #else directives control the code block until the first #elif, #elifdef, #elifndef(since C++23), #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, #elifdef, #elifndef,(since C++23) and #elif directives are ignored. Otherwise, if the specified condition evaluates false, the controlled code block is skipped and the subsequent #else, #elifdef, #elifndef,(since C++23) or #elif directive (if any) is processed. If the subsequent directive is #else, the code block controlled by the #else directive is unconditionally compiled. Otherwise, the #elif, #elifdef, or #elifndef(since C++23) 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, #elifdef, #elifndef,(since C++23) and #else directives. The conditional preprocessing block is terminated by #endif directive.
[edit]Condition evaluation
[edit]#if, #elif
expression may contain unary operators in form defined identifier or defined (identifier). The result is 1 if the identifier was defined as a macro name, otherwise the result is 0.
expression may also contain the following expressions:
The identifiers mentioned above are treated as if they were the names of defined macros in this context. | (since C++17) |
After all macro expansion and evaluation defined and the expressions described above, any identifier which is not a boolean literal is replaced with the number 0 (this includes identifiers that are lexically keywords, but not alternative tokens like and).
Then the expression is evaluated as an integral constant expression.
If the expression evaluates to nonzero value, the controlled code block is included and skipped otherwise.
Note: Until the resolution of CWG issue 1955, #if cond1 ... #elif cond2 is different from #if cond1 ... #else followed by #if cond2 because if cond1 is true, the second #if is skipped and cond2 does not need to be well-formed, while #elif's cond2 must be a valid expression. As of CWG 1955, #elif that leads the skipped code block is also skipped.
[edit]Combined directives
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.
| (since C++23) |
[edit]Notes
While #elifdef and #elifndef directives target C++23, implementations are encouraged to backport them to the older language modes as conforming extensions.
[edit]Example
#define ABCD 2#include <iostream> int main(){ #ifdef ABCDstd::cout<<"1: yes\n";#elsestd::cout<<"1: no\n";#endif #ifndef ABCDstd::cout<<"2: no1\n";#elif ABCD == 2std::cout<<"2: yes\n";#elsestd::cout<<"2: no2\n";#endif #if !defined(DCBA) && (ABCD < 2*4-3)std::cout<<"3: yes\n";#endif // Note that if a compiler does not support C++23's #elifdef/#elifndef// directives then the "unexpected" block (see below) will be selected.#ifdef CPUstd::cout<<"4: no1\n";#elifdef GPUstd::cout<<"4: no2\n";#elifndef RAMstd::cout<<"4: yes\n";// expected block#elsestd::cout<<"4: no!\n";// unexpectedly selects this block by skipping// unknown directives and "jumping" directly// from "#ifdef CPU" to this "#else" block#endif // To fix the problem above we may conditionally define the// macro ELIFDEF_SUPPORTED only if the C++23 directives// #elifdef/#elifndef are supported.#if 0#elifndef UNDEFINED_MACRO#define ELIFDEF_SUPPORTED#else#endif #ifdef ELIFDEF_SUPPORTED#ifdef CPUstd::cout<<"4: no1\n";#elifdef GPUstd::cout<<"4: no2\n";#elifndef RAMstd::cout<<"4: yes\n";// expected block#elsestd::cout<<"4: no3\n";#endif#else // when #elifdef unsupported use old verbose “#elif defined”#ifdef CPUstd::cout<<"4: no1\n";#elif defined GPUstd::cout<<"4: no2\n";#elif !defined RAMstd::cout<<"4: yes\n";// expected block#elsestd::cout<<"4: no3\n";#endif#endif}
Possible output:
1: yes 2: yes 3: yes 4: no! 4: 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 |
|---|---|---|---|
| CWG 1955 | C++98 | failed #elif's expression was required to be valid | failed #elif is skipped |
[edit]See also
C documentation for Conditional inclusion |
