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

From cppreference.com
< cpp‎ | types

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.

`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.

The behavior of a program that adds specializations for any of the templates described on this page is undefined.

### Helper variable templates

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)

## 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, `std::is_nothrow_default_constructible` also checks if the destructor throws because it is effectively noexcept(T()). Same applies to `std::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

### See also

is_constructible is_trivially_constructible is_nothrow_constructible (C++11)(C++11)(C++11) | checks if a type has a constructor for specific arguments (class template)
is_copy_constructible is_trivially_copy_constructible is_nothrow_copy_constructible (C++11)(C++11)(C++11) | checks if a type has a copy constructor (class template)
is_move_constructible is_trivially_move_constructible is_nothrow_move_constructible (C++11)(C++11)(C++11) | checks if a type can be constructed from an rvalue reference (class template)
default_initializable (C++20) | specifies that an object of a type can be default constructed (concept)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/types/is_default_constructible&oldid=152433"