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Showing content from https://en.cppreference.com/w/cpp/language/../error/error_code/../../../cpp/types/is_aggregate.html below:

std::is_aggregate - cppreference.com

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.

[edit] Template parameters [edit] 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 true if T is an aggregate type, false otherwise
(public static member constant) Member functions converts the object to bool, returns value
(public member function) returns value
(public member function) Member types [edit] Notes [edit] Example

#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");
}

[edit] 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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