std::is_default_constructible, std::is_trivially_default_constructible, std::is_nothrow_default_constructible

template< class T >
struct is_default_constructible;

template< class T >
struct is_trivially_default_constructible;

template< class T >
struct is_nothrow_default_constructible;

1) If std::is_constructible<T>::value is true, provides the member constant value equal to true, otherwise value is false.

2) If std::is_trivially_constructible<T>::value is true, provides the member constant value equal to true, otherwise value is false.

3) If std::is_nothrow_constructible<T>::value is true, provides the member constant value equal to true, otherwise value is false.

T shall be a complete type, (possibly cv-qualified) void, or an array of unknown bound. Otherwise, the behavior is undefined.

If an instantiation of a template above depends, directly or indirectly, on an incomplete type, and that instantiation could yield a different result if that type were hypothetically completed, the behavior is undefined.

Helper variable templates

template< class T >
inline constexpr bool is_default_constructible_v = is_default_constructible<T>::value;

template< class T >
inline constexpr bool is_trivially_default_constructible_v = is_trivially_default_constructible<T>::value;

template< class T >
inline constexpr bool is_nothrow_default_constructible_v = is_nothrow_default_constructible<T>::value;

Possible implementation

template< class T>
struct is_default_constructible : std::is_constructible<T> {};
 
template< class T>
struct is_trivially_default_constructible : std::is_trivially_constructible<T> {};
 
template< class T>
struct is_nothrow_default_constructible : std::is_nothrow_constructible<T> {};

Notes

In many implementations, is_nothrow_default_constructible also checks if the destructor throws because it is effectively noexcept(T()). Same applies to is_trivially_default_constructible, which, in these implementations, also requires that the destructor is trivial: GCC bug 51452 LWG issue 2116.

std::is_default_constructible<T> does not test that T x; would compile; it attempts direct-initialization with an empty argument list (see std::is_constructible). Thus, std::is_default_constructible_v<const int> and std::is_default_constructible_v<const int[10]> are true.

Example

#include <iostream>
#include <type_traits>
 
struct Ex1 {
    std::string str; // member has a non-trivial default ctor
};
struct Ex2 {
    int n;
    Ex2() = default; // trivial and non-throwing
};
 
int main() {
    std::cout << std::boolalpha << "Ex1 is default-constructible? "
              << std::is_default_constructible<Ex1>::value << '\n'
              << "Ex1 is trivially default-constructible? "
              << std::is_trivially_default_constructible<Ex1>::value << '\n'
              << "Ex2 is trivially default-constructible? "
              << std::is_trivially_default_constructible<Ex2>::value << '\n'
              << "Ex2 is nothrow default-constructible? "
              << std::is_nothrow_default_constructible<Ex2>::value << '\n';
}

Ex1 is default-constructible? true
Ex1 is trivially default-constructible? false
Ex2 is trivially default-constructible? true
Ex2 is nothrow default-constructible? true

See also