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std::is_aggregate(3) C++ Standard Libary std::is_aggregate(3) NAME std::is_aggregate - std::is_aggregate Synopsis Defined in header <type_traits> template< class T > (since C++17) struct is_aggregate; std::is_aggregate is a UnaryTypeTrait. If T is an aggregate type, provides the member constant value equal true. For any other type, value is false. If T is an incomplete type other than an array type or (possibly cv- qualified) void, the behavior is undefined. If the program adds specializations for std::is_aggregate or std::is_aggregate_v, the behavior is undefined. Template parameters T - a type to check Helper variable template template< class T > (since C++17) inline constexpr bool is_aggregate_v = is_aggregate<T>::value; Inherited from std::integral_constant Member constants value true if T is an aggregate type, false otherwise [static] (public static member constant) Member functions operator bool converts the object to bool, returns value (public member function) operator() returns value (C++14) (public member function) Member types Type Definition value_type bool type std::integral_constant<bool, value> Notes Feature-test macro Value Std Feature __cpp_lib_is_aggregate 201703L (C++17) std::is_agregate Example // Run this code #include <new> #include <type_traits> #include <utility> // constructs a T at the uninitialized memory pointed to by p using // list-initialization for aggregates and non-list initialization oth- erwise template<class T, class... Args> T* construct(T* p, Args&&... args) { if constexpr (std::is_aggregate_v<T>) return ::new (static_cast<void*>(p)) T{std::for- ward<Args>(args)...}; else return ::new (static_cast<void*>(p)) T(std::for- ward<Args>(args)...); } struct A { int x, y; }; struct B { B(int, const char*) {} }; int main() { std::aligned_union_t<1, A, B> storage; [[maybe_unused]] A* a = construct(reinterpret_cast<A*>(&storage), 1, 2); [[maybe_unused]] B* b = construct(reinterpret_cast<B*>(&storage), 1, "hello"); } Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied Behavior as published Correct behavior to The behavior is undefined if T is an The behavior is defined LWG array type but regardless of the 3823 C++17 std::remove_all_extents_t<T> is an incompleteness of incomplete type. std::re- move_all_extents_t<T> as long as T is an array type. http://cppreference.com 2024.06.10 std::is_aggregate(3)
NAME | Synopsis | Template parameters | Member constants | Member functions | Member types | Notes | Example
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