std::is_aggregate

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

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 > 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	Feature
`__cpp_lib_is_aggregate`	201703L	(C++17)	`std::is_agregate`

Example

Run this code

#include <algorithm>
#include <cassert>
#include <cstddef>
#include <new>
#include <string_view>
#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; };
static_assert(std::is_aggregate_v<A>);
 
struct B
{
    int i;
    std::string_view str;
 
    B(int i, std::string_view str) : i(i), str(str) {}
};
static_assert(not std::is_aggregate_v<B>);
 
template <typename... Ts>
using aligned_storage_t = alignas(Ts...) std::byte[std::max({sizeof(Ts)...})];
 
int main()
{
    aligned_storage_t<A, B> storage;
 
    A& a = *construct(reinterpret_cast<A*>(&storage), 1, 2);
    assert(a.x == 1 and a.y == 2);
 
    B& b = *construct(reinterpret_cast<B*>(&storage), 3, "4");
    assert(b.i == 3 and b.str == "4");
}

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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