NAME¶

std::is_aggregate - std::is_aggregate

Synopsis¶

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

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 std::remove_all_extents_t<T> is an incomplete type
other than (possibly cv-qualified) void.

The behavior of a program that adds specializations for is_aggregate or
is_aggregate_v 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: __cpp_lib_is_aggregate

Example¶

// Run this code

#include <type_traits>
#include <new>
#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");
}