View source for cpp/feature test

{{title|Feature testing {{mark since c++20}}}}
{{cpp/feature test/navbar}}

The standard defines a set of [[cpp/preprocessor/replace|preprocessor macros]] corresponding to C++ language and library features introduced in C++11 or later. They are intended as a simple and portable way to detect the presence of said features.

===Attributes===
{{sdsc begin}}
{{sdsc|{{ttb|__has_cpp_attribute(}} {{spar|attribute-token}} {{ttb|)}}}}
{{sdsc end}}

Checks for the support of an [[cpp/language/attributes|attribute]] named by {{spar|attribute-token}} (after macro expansion).

For each standard attribute, it is implementation-defined whether {{tt|__has_cpp_attribute}} expands to the year and month in which the attribute was added to the working draft (see table below) or {{c|0}}. It is recommended that non-zero value is given if and only if the standard attribute causes the implementation to behave as recommended (issuing diagnostic messages, affecting class layout, etc.).

The presence of vendor-specific attributes is determined by a non-zero value.

{{tt|__has_cpp_attribute}} can be expanded in the expression of {{ltt|cpp/preprocessor/conditional|#if}} and {{ltt|cpp/preprocessor/conditional|#elif}}.
It is treated as a defined macro by {{ltt|cpp/preprocessor/conditional|#ifdef}}, {{ltt|cpp/preprocessor/conditional|#ifndef}}{{rev inl|since=c++23|, {{ltt|cpp/preprocessor/conditional|#elifdef}}, {{ltt|cpp/preprocessor/conditional|#elifndef}}}} and {{ltt|cpp/preprocessor/conditional|defined}} but cannot be used anywhere else.

{| class="wikitable sortable"
|-
! {{spar|attribute-token}}
! Attribute
! Value
! <span title="Standard in which the feature is introduced">Standard</span>
! Paper(s)
|-
|{{tt|assume}}
|{{attr|assume}}
|{{c|202207L}}
|{{mark c++23}}
|{{stddoc|P1774R8}}
|-
|{{tt|carries_dependency}}
|{{attr|carries_dependency}}
|{{c|200809L}}
|{{mark c++11}}
|{{stddoc|N2556}}, {{stddoc|N2643}}
|-
|{{tt|deprecated}}
|{{attr|deprecated}}
|{{c|201309L}}
|{{mark c++14}}
|{{stddoc|N3760}}
|-
|{{tt|fallthrough}}
|{{attr|fallthrough}}
|{{c|201603L}}
|{{mark c++17}}
|{{stddoc|P0188R1}}
|-
|{{tt|likely}}
|{{attr|likely}}
|{{c|201803L}}
|{{mark c++20}}
|{{stddoc|P0479R5}}
|-
|{{tt|maybe_unused}}
|{{attr|maybe_unused}}
|{{c|201603L}}
|{{mark c++17}}
|{{stddoc|P0212R1}}
|-
|{{tt|no_unique_address}}
|{{attr|no_unique_address}}
|{{c|201803L}}
|{{mark c++20}}
|{{stddoc|P0840R2}}
|-
| rowspan="2" | {{tt|nodiscard}}
| rowspan="2" | {{attr|nodiscard}}
|{{c|201603L}}
|{{mark c++17}}
|{{stddoc|P0189R1}}
|-
|{{c|201907L}}
|{{mark c++20}}
|{{stddoc|P1301R4}}
|-
|{{tt|noreturn}}
|{{attr|noreturn}}
|{{c|200809L}}
|{{mark c++11}}
|{{stddoc|N2761}}
|-
|{{tt|unlikely}}
|{{attr|likely|unlikely}}
|{{c|201803L}}
|{{mark c++20}}
|{{stddoc|P0479R5}}
|-
! colspan="5" | Total number of attributes: 10 <!-- do not forget to update -->
|}

===Language features===
The following macros are predefined in every translation unit. Each macro expands to an integer literal corresponding to the year and month when the corresponding feature has been included in the working draft.

When a feature changes significantly, the macro will be updated accordingly.

{{include editlink|cpp/utility/lang_feature_macros}}

{{cpp/utility/lang_feature_macros}}

===Library features===
The following macros are defined if the header {{header|version}} or any of the corresponding headers in the table below is included. Each macro expands to an integer literal corresponding to the year and month when the corresponding feature has been included in the working draft.

When a feature changes significantly, the macro will be updated accordingly.

{{include editlink | cpp/utility/lib_feature_macros}}

{{cpp/utility/lib_feature_macros}}

===Example===
====Normal usage====
{{example|code=
#ifdef __has_include                           // Check if __has_include is present
#  if __has_include(<optional>)                // Check for a standard library
#    include <optional>
#  elif __has_include(<experimental/optional>) // Check for an experimental version
#    include <experimental/optional>
#  elif __has_include(<boost/optional.hpp>)    // Try with an external library
#    include <boost/optional.hpp>
#  else                                        // Not found at all
#     error "Missing <optional>"
#  endif
#endif

#ifdef __has_cpp_attribute                      // Check if __has_cpp_attribute is present
#  if __has_cpp_attribute(deprecated)           // Check for an attribute
#    define DEPRECATED(msg) [[deprecated(msg)]]
#  endif
#endif
#ifndef DEPRECATED
#    define DEPRECATED(msg)
#endif

DEPRECATED("foo() has been deprecated") void foo();

#if __cpp_constexpr >= 201304                // Check for a specific version of a feature
#  define CONSTEXPR constexpr
#else
#  define CONSTEXPR inline
#endif

CONSTEXPR int bar(unsigned i)
{
#if __cpp_binary_literals                    // Check for the presence of a feature
    unsigned mask1 = 0b11000000;
    unsigned mask2 = 0b00000111;
#else
    unsigned mask1 = 0xC0;
    unsigned mask2 = 0x07;
#endif
    if ( i & mask1 )
        return 1;
    if ( i & mask2 )
        return 2;
    return 0;
}

int main()
{
}
}}

====Compiler Features Dump====
{{include editlink | cpp/compiler_support/dump}}

<!--{{cot|Compiler Features Dump Example}}-->
{{cpp/compiler_support/dump}}
<!--{{cob}}-->

===See also===
{{dsc begin}}
{{dsc | [[cpp/utility/feature_test|'''Library feature-test macros''']] {{mark c++20}} | defined in the header {{header|version}} }}
{{dsc see cpp | cpp/symbol_index/macro | Macro Symbol Index | nomono=true}}
{{dsc end}}

===External links===
{{elink begin}}
{{elink|num=1|[https://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations The official document on Feature-Test Recommendations]}}
{{elink|num=2|[https://github.com/makelinux/examples/blob/develop/cpp/features.cpp Source code to dump compiler features]}}
{{elink end}}

{{langlinks|es|ja|ru|zh}}