# std::is_copy_constructible, std::is_trivially_copy_constructible, std::is_nothrow_copy_constructible

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< cpp‎ | types

Defined in header `<type_traits>` |  |
template< class T > struct is_copy_constructible; | (1) | (since C++11)
template< class T > struct is_trivially_copy_constructible; | (2) | (since C++11)
template< class T > struct is_nothrow_copy_constructible; | (3) | (since C++11)

1) If `T` is not a referenceable type (i.e., possibly cv-qualified void or a function type with a cv-qualifier-seq or a ref-qualifier), provides a member constant `value` equal to false. Otherwise, provides a member constant `value` equal to std::is_constructible <T, const T&>::value.

2) Same as (1), but uses std::is_trivially_constructible <T, const T&>.

3) Same as (1), but uses std::is_nothrow_constructible <T, const T&>.

`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_copy_constructible_v = is_copy_constructible<T>::value; |  | (since C++17)
template< class T > inline constexpr bool is_trivially_copy_constructible_v = is_trivially_copy_constructible<T>::value; |  | (since C++17)
template< class T > inline constexpr bool is_nothrow_copy_constructible_v = is_nothrow_copy_constructible<T>::value; |  | (since C++17)

## Inherited from std::integral_constant

### Member constants

value [static] | true if `T` is copy-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_copy_constructible :
        std::is_constructible<T, typename std::add_lvalue_reference<
            typename std::add_const<T>::type>::type>> {};

    template<class T>
    struct is_trivially_copy_constructible :
        std::is_trivially_constructible<T, typename std::add_lvalue_reference<
            typename std::add_const<T>::type>::type>> {};

    template<class T>
    struct is_nothrow_copy_constructible :
        std::is_nothrow_constructible<T, typename std::add_lvalue_reference<
            typename std::add_const<T>::type>::type>> {};

### Notes

In many implementations, `is_nothrow_copy_constructible` also checks if the destructor throws because it is effectively noexcept(T(arg)). Same applies to `is_trivially_copy_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>

    struct Ex1 {
        std::string str; // member has a non-trivial copy ctor
    };
    struct Ex2 {
        int n;
        Ex2(const Ex2&) = default; // trivial and non-throwing
    };

    int main() {
        std::cout << std::boolalpha << "Ex1 is copy-constructible? "
                  << std::is_copy_constructible<Ex1>::value << '\n'
                  << "Ex1 is trivially copy-constructible? "
                  << std::is_trivially_copy_constructible<Ex1>::value << '\n'
                  << "Ex2 is trivially copy-constructible? "
                  << std::is_trivially_copy_constructible<Ex2>::value << '\n'
                  << "Ex2 is nothrow copy-constructible? "
                  << std::is_nothrow_copy_constructible<Ex2>::value << '\n';
    }

Output:

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

### See also

is_constructibleis_trivially_constructibleis_nothrow_constructible (C++11)(C++11)(C++11) | checks if a type has a constructor for specific arguments (class template)
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_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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