std::is_copy_constructible,std::is_trivially_copy_constructible, (3) - Linux Manuals

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

NAME

std::is_copy_constructible,std::is_trivially_copy_constructible, - std::is_copy_constructible,std::is_trivially_copy_constructible,

Synopsis

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

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

Inherited from std::integral_constant

Member constants

value    true if T is copy-constructible , false otherwise
[static] (public static member constant)

Member functions

operator bool converts the object to bool, returns value
        (public member function)
operator()    returns value
(C++14)       (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