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

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Type properties
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Metafunctions
(C++17)
Supported operations
is_copy_constructibleis_trivially_copy_constructibleis_nothrow_copy_constructible
(C++11)(C++11)(C++11)
Relationships and property queries
(C++11)
(C++11)
(C++11)
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Type modifications
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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)

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.

Contents

[edit] Helper variable templates

template< class T >
constexpr bool is_copy_constructible_v = is_copy_constructible<T>::value;
(since C++17)
template< class T >
constexpr bool is_trivially_copy_constructible_v = is_trivially_copy_constructible<T>::value;
(since C++17)
template< class T >
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>

[edit] 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>> {};

[edit] 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.

[edit] 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

[edit] See also

checks if a type has a constructor for specific arguments
(class template) [edit]
checks if a type has a default constructor
(class template) [edit]
checks if a type can be constructed from an rvalue reference
(class template) [edit]
variable template alias of std::is_copy_constructible::value
(variable template) [edit]
variable template alias of std::is_trivially_copy_constructible::value
(variable template) [edit]
variable template alias of std::is_nothrow_copy_constructible::value
(variable template) [edit]