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Feature testing (C++20)

From cppreference.com
< cpp

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.

Contents

[edit] Attributes

__has_cpp_attribute( attribute-token )

Checks for the presence of an attribute.

For standard attributes, it will expand to the year and month in which the attribute was added to the working draft (see table below), 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 and defined but cannot be used anywhere else.

attribute-token Attribute Value Standard
assert [[assert]] 201806L (C++20)
carries_dependency [[carries_dependency]] 200809L (C++11)
deprecated [[deprecated]] 201309L (C++14)
ensures [[ensures]] 201806L (C++20)
expects [[expects]] 201806L (C++20)
fallthrough [[fallthrough]] 201603L (C++17)
likely [[likely]] 201803L (C++20)
maybe_unused [[maybe_unused]] 201603L (C++17)
no_unique_address [[no_unique_address]] 201803L (C++20)
nodiscard [[nodiscard]] 201603L (C++17)
noreturn [[noreturn]] 200809L (C++11)
unlikely [[unlikely]] 201803L (C++20)

[edit] 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.

Macro name Feature Value Std
__cpp_aggregate_bases Extension to aggregate initialization 201603L (C++17)
__cpp_aggregate_nsdmi Member initializers and aggregates 201304L (C++14)
__cpp_alias_templates Template aliases 200704L (C++11)
__cpp_aligned_new Dynamic memory allocation for over-aligned data 201606L (C++17)
__cpp_attributes Attributes 200809L (C++11)
__cpp_binary_literals Binary literals in the C++ core language 201304L (C++14)
__cpp_capture_star_this Lambda capture of *this by value as [=,*this] 201603L (C++17)
__cpp_char8_t char8_t 201811L (C++20)
__cpp_conditional_explicit explicit(bool) 201806L (C++20)
__cpp_constexpr constexpr 200704L (C++11)
Relaxing constraints on constexpr functions / constexpr member functions and implicit const 201304L (C++14)
Constexpr lambda 201603L (C++17)
__cpp_decltype decltype 200707L (C++11)
__cpp_decltype_auto Return type deduction for normal functions 201304L (C++14)
__cpp_deduction_guides Template argument deduction for class templates 201703L (C++17)
__cpp_delegating_constructors Delegating constructors 200604L (C++11)
__cpp_enumerator_attributes Attributes for enumerators 201411L (C++17)
__cpp_fold_expressions Fold expressions 201603L (C++17)
__cpp_generic_lambdas Generic (polymorphic) lambda expressions 201304L (C++14)
__cpp_guaranteed_copy_elision Guaranteed copy elision through simplified value categories 201606L (C++17)
__cpp_hex_float Hexadecimal floating literals 201603L (C++17)
__cpp_if_constexpr constexpr if 201606L (C++17)
__cpp_impl_destroying_delete Destroying operator delete (compiler support) 201806L (C++20)
__cpp_impl_three_way_comparison Three-way comparison (compiler support) 201711L (C++20)
__cpp_inheriting_constructors Inheriting constructors 200802L (C++11)
Rewording inheriting constructors 201511L (C++17)
__cpp_init_captures Generalized lambda-capture 201304L (C++14)
__cpp_initializer_lists Initializer lists 200806L (C++11)
__cpp_inline_variables Inline variables 201606L (C++17)
__cpp_lambdas Lambda expressions 200907L (C++11)
__cpp_namespace_attributes Attributes for namespaces 201411L (C++17)
__cpp_noexcept_function_type Make exception specifications be part of the type system 201510L (C++17)
__cpp_nontype_template_args Allow constant evaluation for all non-type template arguments 201411L (C++17)
__cpp_nontype_template_parameter_auto Declaring non-type template parameters with auto 201606L (C++17)
__cpp_nontype_template_parameter_class Class types in non-type template parameters 201806L (C++20)
__cpp_nsdmi Non-static data member initializers 200809L (C++11)
__cpp_range_based_for Range-based for loop 200907L (C++11)
Generalized range-based for loop 201603L (C++17)
__cpp_raw_strings Raw string literals 200710L (C++11)
__cpp_ref_qualifiers ref-qualifiers 200710L (C++11)
__cpp_return_type_deduction Return type deduction for normal functions 201304L (C++14)
__cpp_rvalue_references Rvalue reference 200610L (C++11)
__cpp_sized_deallocation Sized deallocation 201309L (C++14)
__cpp_static_assert static_assert 200410L (C++11)
Extended static_assert 201411L (C++17)
__cpp_structured_bindings structured bindings 201606L (C++17)
__cpp_template_template_args Matching of template template-arguments 201611L (C++17)
__cpp_threadsafe_static_init Dynamic initialization and destruction with concurrency 200806L (C++11)
__cpp_unicode_characters New character types (char16_t and char32_t) 200704L (C++11)
__cpp_unicode_literals Unicode string literals 200710L (C++11)
__cpp_user_defined_literals User-defined literals 200809L (C++11)
__cpp_variable_templates Variable templates 201304L (C++14)
__cpp_variadic_templates Variadic templates 200704L (C++11)
__cpp_variadic_using Pack expansions in using-declarations 201611L (C++17)

[edit] Library features

The following macros are defined if the 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.

Macro name Feature Value Header Std
__cpp_lib_addressof_constexpr constexpr std::addressof 201603L <memory> (C++17)
__cpp_lib_allocator_traits_is_always_equal std::allocator_traits::is_always_equal 201411L <memory> <scoped_allocator> <string> <deque> <forward_list> <list> <vector> <map> <set> <unordered_map> <unordered_set> (C++17)
__cpp_lib_any std::any 201606L <any> (C++17)
__cpp_lib_apply std::apply 201603L <tuple> (C++17)
__cpp_lib_array_constexpr Adding constexpr modifiers to std::reverse_iterator, std::move_iterator, std::array and range access 201603L <iterator> <array> (C++17)
__cpp_lib_as_const std::as_const 201510L <utility> (C++17)
__cpp_lib_atomic_is_always_lock_free constexpr atomic<T>::is_always_lock_free 201603L <atomic> (C++17)
__cpp_lib_atomic_ref std::atomic_ref 201806L <atomic> (C++20)
__cpp_lib_bind_front std::bind_front <functional> (C++20)
__cpp_lib_bit_­cast std::bit_cast 201806L <bit> (C++20)
__cpp_lib_bool_constant std::bool_constant 201505L <type_traits> (C++17)
__cpp_lib_boyer_moore_searcher Searchers 201603L <functional> (C++17)
__cpp_lib_byte A byte type definition 201603L <cstddef> (C++17)
__cpp_lib_chrono rounding functions for std::chrono::duration and std::chrono::time_point 201510L <chrono> (C++17)
making all the member functions of std::chrono::duration and std::chrono::time_point constexpr 201611L <chrono> (C++17)
__cpp_lib_chrono_udls User-defined literals for time types 201304L <chrono> (C++14)
__cpp_lib_clamp An algorithm to "clamp" a value between a pair of boundary values (std::clamp) 201603L <algorithm> (C++17)
__cpp_lib_complex_udls User-defined Literals for std::complex 201309L <complex> (C++14)
__cpp_­lib_­concepts Standard library concepts 201806L <concepts> (C++20)
__cpp_lib_constexpr_misc Constexpr for miscellaneous facilities 201811L <array> <functional> <iterator> <string_view> <tuple> <utility> (C++20)
__cpp_­lib_­constexpr_­swap_­algorithms Constexpr for swap and swap related functions 201806L <algorithm> (C++20)
__cpp_lib_destroying_delete Destroying operator delete (library support) 201806L <new> (C++20)
__cpp_lib_enable_shared_from_this Re-enabling shared_from_this 201603L <memory> (C++17)
__cpp_lib_exchange_function exchange() utility function 201304L <utility> (C++14)
__cpp_lib_execution Execution policies 201603L <execution> (C++17)
__cpp_lib_filesystem Filesystem library 201703L <filesystem> (C++17)
__cpp_lib_gcd_lcm std::gcd, std::lcm 201606L <numeric> (C++17)
__cpp_lib_generic_associative_lookup Adding heterogeneous comparison lookup to associative containers 201304L <map> <set> (C++14)
__cpp_lib_generic_unordered_lookup Adding heterogeneous comparison lookup to unordered associative containers <unordered_map> <unordered_set> (C++20)
__cpp_lib_hardware_interference_size constexpr std::hardware_{constructive, destructive}_interference_size 201703L <new> (C++17)
__cpp_lib_has_unique_object_representations std::has_unique_object_representations 201606L <type_traits> (C++17)
__cpp_lib_hypot 3-argument overload of std::hypot 201603L <cmath> (C++17)
__cpp_lib_incomplete_container_elements Minimal incomplete type support for standard containers 201505L <forward_list> <list> <vector> (C++17)
__cpp_lib_integer_sequence Compile-time integer sequences 201304L <utility> (C++14)
__cpp_lib_integral_constant_callable std::integral_constant::operator() 201304L <type_traits> (C++14)
__cpp_lib_invoke std::invoke function template 201411L <functional> (C++17)
__cpp_lib_is_aggregate std::is_aggregate type trait 201703L <type_traits> (C++17)
__cpp_lib_is_final std::is_final 201402L <type_traits> (C++14)
__cpp_lib_is_invocable std::is_invocable, std::invoke_result 201703L <type_traits> (C++17)
__cpp_lib_is_null_pointer std::is_null_pointer 201309L <type_traits> (C++14)
__cpp_lib_is_swappable [nothrow-]swappable traits 201603L <type_traits> (C++17)
__cpp_lib_launder Core Issue 1776: Replacement of class objects containing reference members (std::launder) 201606L <new> (C++17)
__cpp_­lib_­list_­remove_­return_­type Change the return type of the remove(), remove_if() and unique() members of std::forward_list and std::list 201806L <forward_list> <list> (C++20)
__cpp_lib_logical_traits Logical operator type traits 201510L <type_traits> (C++17)
__cpp_lib_make_from_tuple std::make_from_tuple: apply for construction 201606L <tuple> (C++17)
__cpp_lib_make_reverse_iterator std::make_reverse_iterator 201402L <iterator> (C++14)
__cpp_lib_make_unique std::make_unique 201304L <memory> (C++14)
__cpp_lib_map_try_emplace std::map::try_emplace, std::map::insert_or_assign 201411L <map> (C++17)
__cpp_lib_math_special_functions Mathematical special functions for C++17 201603L <cmath> (C++17)
__cpp_lib_memory_resource std::pmr::memory_resource 201603L <memory_resource> (C++17)
__cpp_lib_node_extract Splicing maps and sets (std::map::extract, std::map::merge, std::map::insert(node_type), etc) 201606L <map> <set> <unordered_map> <unordered_set> (C++17)
__cpp_lib_nonmember_container_access Non-member size() and more (uniform container access) 201411L <iterator> <array> <deque> <forward_list> <list> <map> <regex> <set> <string> <unordered_map> <unordered_set> <vector> (C++17)
__cpp_lib_not_fn std::not_fn 201603L <functional> (C++17)
__cpp_lib_null_iterators Null LegacyForwardIterators 201304L <iterator> (C++14)
__cpp_lib_optional std::optional 201606L <optional> (C++17)
__cpp_lib_parallel_algorithm Adopt the Parallelism TS for C++17 201603L <algorithm> <numeric> (C++17)
__cpp_lib_quoted_string_io std::quoted 201304L <iomanip> (C++14)
__cpp_lib_raw_memory_algorithms Extending memory management tools 201606L <memory> (C++17)
__cpp_lib_result_of_sfinae std::result_of and SFINAE 201210L <functional> <type_traits> (C++14)
__cpp_lib_robust_nonmodifying_seq_ops Making non-modifying sequence operations more robust (two-range overloads for std::mismatch, std::equal and std::is_permutation) 201304L <algorithm> (C++14)
__cpp_lib_sample std::sample 201603L <algorithm> (C++17)
__cpp_lib_scoped_lock Variadic std::lock_guard (std::scoped_lock) 201703L <mutex> (C++17)
__cpp_lib_shared_mutex std::shared_mutex (untimed) 201505L <shared_mutex> (C++17)
__cpp_lib_shared_ptr_arrays std::shared_ptr<T[]> 201611L <memory> (C++17)
__cpp_lib_shared_ptr_weak_type shared_ptr::weak_type 201606L <memory> (C++17)
__cpp_lib_shared_timed_mutex Rename shared_mutex to shared_timed_mutex 201402L <shared_mutex> (C++14)
__cpp_lib_string_udls User-defined literals for string types 201304L <string> (C++14)
__cpp_lib_string_view std::string_view 201606L <string> <string_view> (C++17)
__cpp_lib_three_way_comparison Three-way comparison (library support) 201711L <compare> (C++20)
__cpp_lib_to_chars Elementary string conversions (std::to_chars, std::from_chars) 201611L <charconv> (C++17)
__cpp_lib_transformation_trait_aliases TransformationTraits Redux 201304L <type_traits> (C++14)
__cpp_lib_transparent_operators Transparent operator functors (std::less<> et al) 201210L <functional> (C++14)
Making std::owner_less more flexible (std::owner_less<void>) 201510L <memory> <functional> (C++17)
__cpp_lib_tuple_element_t std::tuple_element_t 201402L <tuple> (C++14)
__cpp_lib_tuples_by_type Addressing tuples by type 201304L <utility> <tuple> (C++14)
__cpp_lib_type_trait_variable_templates Type traits variable templates (std::is_void_v, etc) 201510L <type_traits> (C++17)
__cpp_lib_uncaught_exceptions std::uncaught_exceptions 201411L <exception> (C++17)
__cpp_lib_unordered_map_try_emplace std::unordered_map::try_emplace, std::unordered_map::insert_or_assign 201411L <unordered_map> (C++17)
__cpp_lib_variant std::variant: a type-safe union for C++17 201606L <variant> (C++17)
__cpp_lib_void_t std::void_t 201411L <type_traits> (C++17)

[edit] Example

#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()
{
}


[edit] See Also

Standing Document 6 The official document on Feature Test Recommendations