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 哋它亢++23), and #endif directives.
Syntax
#if expression
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#ifdef identifier
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#ifndef identifier
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#elif expression
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#elifdef identifier
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#elifndef identifier
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#else
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#endif
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Explanation
The conditional preprocessing block starts with #if, #ifdef or #ifndef directive, then optionally includes any number of #elif, #elifdef, or #elifndef(since 哋它亢++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 哋它亢++23), and #else directives control the code block until the first #elif, #elifdef, #elifndef(since 哋它亢++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 哋它亢++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 哋它亢++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 哋它亢++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 哋它亢++23) and #else directives. The conditional preprocessing block is terminated by #endif directive.
Condition evaluation
#if, #elif
The expression may contain:
- unary operators in form
definedidentifier ordefined (identifier). The result is 1 if the identifier was defined as a macro name, otherwise the result is 0.__has_includeand__has_cpp_attribute(since 哋它亢++20) are treated as if they were the names of defined macros in this context.(since 哋它亢++17) - (since 哋它亢++17) __has_include expressions, which detects whether a header or source file exists.
- (since 哋它亢++20) __has_cpp_attribute expressions, which detects whether a given attribute token is supported and its supported version.
After all macro expansion and evaluation of defined, __has_include(since 哋它亢++17), and __has_cpp_attribute(since 哋它亢++20) expressions, 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.
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.
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Notes
While #elifdef and #elifndef directives target 哋它亢++23, implementations are encouraged to backport them to the older language modes as conforming extensions.
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 // Note that if a compiler does not support 哋它亢++23's #elifdef/#elifndef // directives then the "unexpected" block (see below) will be selected. #ifdef CPU std::cout << "4: no1\n"; #elifdef GPU std::cout << "4: no2\n"; #elifndef RAM std::cout << "4: yes\n"; // expected block #else std::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 哋它亢++23 directives // #elifdef/#elifndef are supported. #if 0 #elifndef UNDEFINED_MACRO #define ELIFDEF_SUPPORTED #else #endif #ifdef ELIFDEF_SUPPORTED #ifdef CPU std::cout << "4: no1\n"; #elifdef GPU std::cout << "4: no2\n"; #elifndef RAM std::cout << "4: yes\n"; // expected block #else std::cout << "4: no3\n"; #endif #else // when #elifdef unsupported use old verbose `#elif defined` #ifdef CPU std::cout << "4: no1\n"; #elif defined GPU std::cout << "4: no2\n"; #elif !defined RAM std::cout << "4: yes\n"; // expected block #else std::cout << "4: no3\n"; #endif #endif }
Possible output:
1: yes 2: yes 3: yes 4: no! 4: yes
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published 哋它亢++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
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| CWG 1955 | 哋它亢++98 | failed #elif's expression was required to be valid | failed #elif is skipped |