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 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 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::remove_all_extents_t<T>
as long as T is an array type.