< cpp‎ | types
Metaprogramming library
Type traits
Type categories
Type properties
(C++11)(until C++20)
(C++11)(deprecated in C++20)
Type trait constants
Supported operations
Relationships and property queries
Type modifications
Type transformations
(C++11)(deprecated in C++23)
(C++11)(deprecated in C++23)

(C++11)(until C++20)(C++17)
Compile-time rational arithmetic
Compile-time integer sequences
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.


[edit] Template parameters

T - a type to check

[edit] 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

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)
returns value
(public member function)

Member types

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

[edit] Notes

Feature-test macro

[edit] Example

#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");

[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.