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

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

NAME

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

Inherited from std::integral_constant

Member constants

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

Member functions

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

operator() returns value
              (public member function)
(C++14)

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