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std::is_aggregate

Defined in header `<type_traits>`
template< class T > struct is_aggregate;		(since C++17)

Checks if T is an aggregate type. The member constant value is equal to true if T is an aggregate type and false otherwise.

The behavior is undefined if T is an incomplete type other than an array type or (possibly cv-qualified) void.

The behavior of a program that adds specializations for is_aggregate or is_aggregate_v is undefined.

Template parameters

a type to check

Helper variable template

template< class T >
inline constexpr bool is_aggregate_v = is_aggregate<T>::value;

(since C++17)

Inherited from std::integral_constant

Member constants

value

[static]

true if T is an aggregate type, false otherwise
(public static member constant)

Member functions

operator bool	converts the object to bool, returns `value` (public member function)
operator() (C++14)	returns `value` (public member function)

Member types

Type	Definition
`value_type`	bool
`type`	std::integral_constant<bool, value>

Notes

Feature-test macro	Value	Std	Comment
`__cpp_lib_is_aggregate`	201703L	(C++17)	`std::is_agregate`

Example

#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 otherwise
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::forward<Args>(args)...};
    else
        return ::new (static_cast<void*>(p)) T(std::forward<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 retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 3823	C++17	The behavior is undefined if `T` is an array type but `std::remove_all_extents_t<T>` is an incomplete type.	The behavior is defined regardless of the incompleteness of `std::remove_all_extents_t<T>` as long as `T` is an array type.

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