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# 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) 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.

### [edit] 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)

[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>

### [edit] 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> {};

### [edit] 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.

### [edit] 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

### [edit] 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) [edit]
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) [edit]
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) [edit]

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