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

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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)

Type trait | The value of the member constant `value`
--- | ---
`T` is a referenceable type | `T` is not a referenceable type
(1) | std::is_constructible <T, const T&>::value | false
(2) | std::is_trivially_constructible <T, const T&>::value
(3) | std::is_nothrow_constructible <T, const 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.

### 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 <string>
    #include <type_traits>

    struct S1
    {
        std::string str; // member has a non-trivial copy constructor
    };
    static_assert(std::is_copy_constructible_v<S1>);
    static_assert(!std::is_trivially_copy_constructible_v<S1>);

    struct S2
    {
        int n;
        S2(const S2&) = default; // trivial and non-throwing
    };
    static_assert(std::is_trivially_copy_constructible_v<S2>);
    static_assert(std::is_nothrow_copy_constructible_v<S2>);

    struct S3
    {
        S3(const S3&) = delete; // explicitly deleted
    };
    static_assert(!std::is_copy_constructible_v<S3>);

    struct S4
    {
        S4(S4&) {}; // cannot bind const, hence not a copy-constructible
    };
    static_assert(!std::is_copy_constructible_v<S4>);

    int main() {}

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2196 | C++11 | the behavior was unclear if const T& cannot be formed | the value produced is false in this case

### 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)
copy_constructible (C++20) | specifies that an object of a type can be copy constructed and move constructed (concept)

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