Namespaces
Variants
Views
Actions

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

From cppreference.com
< cpp‎ | types
 
 
Metaprogramming library
Type traits
Type categories
(C++11)
(C++14)  
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
Type properties
(C++11)
(C++11)
(C++14)
(C++11)
(C++11)(until C++20*)
(C++11)(deprecated in C++20)
(C++11)
Type trait constants
Metafunctions
(C++17)
Supported operations
Relationships and property queries
Type modifications
(C++11)(C++11)(C++11)
Type transformations
(C++11)(deprecated in C++23)
(C++11)(deprecated in C++23)
(C++11)
(C++11)
(C++17)

(C++11)(until C++20*)(C++17)
Compile-time rational arithmetic
Compile-time integer sequences
 
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.

Contents

[edit] 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>

[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 <string>
#include <type_traits>
 
struct S1
{
    std::string str; // member has a non-trivial copy constructor
};
static_assert(std::is_copy_constructible_v<S1> == true);
static_assert(std::is_trivially_copy_constructible_v<S1> == false);
 
struct S2
{
    int n;
    S2(const S2&) = default; // trivial and non-throwing
};
static_assert(std::is_trivially_copy_constructible_v<S2> == true);
static_assert(std::is_nothrow_copy_constructible_v<S2> == true);
 
int main() {}

[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]
specifies that an object of a type can be copy constructed and move constructed
(concept) [edit]