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    Feature testing (since C++20)

    From cppreference.com
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    Feature testing
     

    The standard defines a set of 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

    __has_cpp_attribute( attribute-token )

    Checks for the support of an attribute named by attribute-token (after macro expansion).

    For each standard attribute, it is implementation-defined whether __has_cpp_attribute expands to the value given in the table below (which is the year and month in which the attribute was added to the working draft) or 0. It will expand to given value in the table 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.

    __has_cpp_attribute can be expanded in the expression of #if and #elif. It is treated as a defined macro by #ifdef, #ifndef, #elifdef, #elifndef(since C++23) and defined but cannot be used anywhere else.

    attribute-token Attribute Value Std Paper(s)
    assume [[assume]] 202207L (C++23) P1774R8
    carries_dependency [[carries_dependency]] 200809L (C++11)
    (until C++26)     		N2556
    N2643
    P3475R2
    deprecated [[deprecated]] 201309L (C++14) N3760
    fallthrough [[fallthrough]] 201603L (C++17) P0188R1
    indeterminate [[indeterminate]] 202403L (C++26) P2795R5
    likely [[likely]] 201803L (C++20) P0479R5
    maybe_unused [[maybe_unused]] 201603L (C++17) P0212R1
    no_unique_address [[no_unique_address]] 201803L (C++20) P0840R2
    nodiscard [[nodiscard]] 201603L (C++17) P0189R1
    [[nodiscard]] with reason 201907L (C++20) P1301R4
    noreturn [[noreturn]] 200809L (C++11) N2761
    unlikely [[unlikely]] 201803L (C++20) P0479R5
    Total number of attributes: 11

    Language features

    The following macros can be used to detect whether a language feature is implemented by the current implementation. They 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.

    [edit]
    Macro name Feature Value Std Paper(s)
    __cpp_aggregate_bases Aggregate classes with base classes 201603L (C++17) P0017R1
    __cpp_aggregate_nsdmi Aggregate classes with default member initializers 201304L (C++14) N3653
    __cpp_aggregate_paren_init Aggregate initialization in the form of direct initialization 201902L (C++20) P0960R3
    __cpp_alias_templates Alias templates 200704L (C++11) N2258
    __cpp_aligned_new Dynamic memory allocation for over-aligned data 201606L (C++17) P0035R4
    __cpp_attributes Attributes 200809L (C++11) N2761
    __cpp_auto_cast Casting an expression to a prvalue (auto(x))