# std::is_constructible, std::is_trivially_constructible, std::is_nothrow_constructible

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< cpp‎ | types

Defined in header `<type_traits>` |  |
template< class T, class... Args > struct is_constructible; | (1) | (since C++11)
template< class T, class... Args > struct is_trivially_constructible; | (2) | (since C++11)
template< class T, class... Args > struct is_nothrow_constructible; | (3) | (since C++11)

1) If `T` is an object or reference type and the variable definition T obj( std::declval <Args>()...); is well-formed, provides the member constant `value` equal to true. In all other cases, `value` is false.

For the purposes of this check, the variable definition is never interpreted as a function declaration, and the use of std::declval is not considered an odr-use. Access checks are performed as if from a context unrelated to `T` and any of the types in `Args`. Only the validity of the immediate context of the variable definition is considered.

2) same as 1), but the variable definition does not call any operation that is not trivial. For the purposes of this check, the call to std::declval is considered trivial.

3) same as 1), but the variable definition is `noexcept`.

`T` and all types in the parameter pack `Args` shall each 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, class... Args > inline constexpr bool is_constructible_v = is_constructible<T, Args...>::value; |  | (since C++17)
template< class T, class... Args > inline constexpr bool is_trivially_constructible_v = is_trivially_constructible<T, Args...>::value; |  | (since C++17)
template< class T, class... Args > inline constexpr bool is_nothrow_constructible_v = is_nothrow_constructible<T, Args...>::value; |  | (since C++17)

## Inherited from std::integral_constant

### Member constants

value [static] | true if `T` is constructible from `Args...` , 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>

### Notes

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

### Example

Run this code

    #include <iostream>
    #include <type_traits>

    class Foo {
        int v1;
        double v2;
     public:
        Foo(int n) : v1(n), v2() {}
        Foo(int n, double f) noexcept : v1(n), v2(f) {}
    };

    int main() {
        std::cout << "Foo is ...\n" << std::boolalpha
                  << "\tTrivially-constructible from const Foo&? "
                  << std::is_trivially_constructible<Foo, const Foo&>::value << '\n'
                  << "\tTrivially-constructible from int? "
                  << std::is_trivially_constructible<Foo, int>::value << '\n'
                  << "\tConstructible from int? "
                  << std::is_constructible<Foo, int>::value << '\n'
                  << "\tNothrow-constructible from int? "
                  << std::is_nothrow_constructible<Foo, int>::value << '\n'
                  << "\tNothrow-constructible from int and double? "
                  << std::is_nothrow_constructible<Foo, int, double>::value << '\n';
    }

Output:

    Foo is ...
            Trivially-constructible from const Foo&? true
            Trivially-constructible from int? false
            Constructible from int? true
            Nothrow-constructible from int? false
            Nothrow-constructible from int and double? true

### See also

is_default_constructibleis_trivially_default_constructibleis_nothrow_default_constructible (C++11)(C++11)(C++11) | checks if a type has a default constructor (class template)
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)
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)

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