Docs4dev
Docs
cpp / latest / types / is_copy_constructible.html

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

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.

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_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

#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

(C++11)(C++11)(C++11)
checks if a type has a constructor for specific arguments
(class template)
(C++11)(C++11)(C++11)
checks if a type has a default constructor
(class template)
(C++11)(C++11)(C++11)
checks if a type can be constructed from an rvalue reference
(class template)
(C++20)
specifies that an object of a type can be copy constructed and move constructed
(concept)

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
https://en.cppreference.com/w/cpp/types/is_copy_constructible