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

Defined in header `<type_traits>`
template< class T > struct is_default_constructible;	(1)	(since C++11)
template< class T > struct is_trivially_default_constructible;	(2)	(since C++11)
template< class T > struct is_nothrow_default_constructible;	(3)	(since C++11)

1) Provides the member constant value equal to std::is_constructible<T>::value.

2) Provides the member constant value equal to std::is_trivially_constructible<T>::value.

3) Provides the member constant value equal to std::is_nothrow_constructible<T>::value.

If T is not a complete type, (possibly cv-qualified) void, or an array of unknown bound, 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.

If the program adds specializations for any of the templates described on this page, the behavior is undefined.

template< class T > inline constexpr bool is_default_constructible_v = is_default_constructible<T>::value;		(since C++17)
template< class T > inline constexpr bool is_trivially_default_constructible_v = is_trivially_default_constructible<T>::value;		(since C++17)
template< class T > inline constexpr bool is_nothrow_default_constructible_v = is_nothrow_default_constructible<T>::value;		(since C++17)

[edit]

Inherited from std::integral_constant

Member constants

value

[static]

true if T is default-constructible , false otherwise
(public static member constant)

Member functions

operator bool	converts the object to bool, returns value (public member function)
operator() (C++14)	returns value (public member function)

Member types

Type	Definition
`value_type`	bool
`type`	std::integral_constant<bool, 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

Run this code

#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';
}

Output:

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

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External libraries

is_constructibleis_trivially_constructibleis_nothrow_constructible (C++11)(C++11)(C++11)	checks if a type has a constructor for specific arguments (class template) [edit]
is_copy_constructibleis_trivially_copy_constructibleis_nothrow_copy_constructible (C++11)(C++11)(C++11)	checks if a type has a copy constructor (class template) [edit]
is_move_constructibleis_trivially_move_constructibleis_nothrow_move_constructible (C++11)(C++11)(C++11)	checks if a type can be constructed from an rvalue reference (class template) [edit]
default_initializable (C++20)	specifies that an object of a type can be default constructed (concept) [edit]

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

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