Conditional inclusion

The preprocessor supports conditional compilation of parts of source file. This behavior is controlled by #if, #ifdef, #ifndef, #else , #elif, #elifdef, #elifndef (since C++23) , and #endif directives.

Syntax

1	#if		expression
2	#ifdef		identifier
3	#ifndef		identifier
4	#elif		expression
5	#elifdef		identifier	(since C++23)
6	#elifndef		identifier	(since C++23)
7	#else
8	#endif

Explanation

• The conditional preprocessing block starts with #if, #defif 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, #defif 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, #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.

Condition evaluation

#if, #elif

The 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. __has_include and __has_cpp_attribute  (since C++20) are treated as if they were the names of defined macros in this context. (since C++17)
• __has_include expressions, which detects whether a header or source file exists (since C++17).
• __has_cpp_attribute expressions, which detects whether a given attribute token is supported and its supported version. (since C++20)

After all macro expansion and evaluation of defined, __has_include (since C++17), and __has_cpp_attribute (since C++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.
• #elifdef identifier is essentially equivalent to #elif defined identifier.  (since C++23)
• #elifndef identifier is essentially equivalent to #elif !defined identifier.  (since C++23)

Notes

While #elifdef and #elifndef directives target (C++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 C++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 C++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 Result

1: yes
2: yes
3: yes
4: no!
4: yes

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