std::is_default_constructible,std::is_trivially_default_constructible,std::is_nothrow_default_constructible (3) - Linux Manuals

std::is_default_constructible,std::is_trivially_default_constructible,std::is_nothrow_default_constructible: std::is_default_constructible,std::is_trivially_default_constructible,std::is_nothrow_default_constructible

NAME

std::is_default_constructible,std::is_trivially_default_constructible,std::is_nothrow_default_constructible - std::is_default_constructible,std::is_trivially_default_constructible,std::is_nothrow_default_constructible

Synopsis

Defined in header <type_traits>
template< class T > (1) (since C++11)
struct is_default_constructible;
template< class T > (2) (since C++11)
struct is_trivially_default_constructible;
template< class T > (3) (since C++11)
struct is_nothrow_default_constructible;

1) If std::is_constructible<T>::value is true, provides the member constant value equal to true, otherwise value is false.
2) If std::is_trivially_constructible<T>::value is true, provides the member constant value equal to true, otherwise value is false.
3) If std::is_nothrow_constructible<T>::value is true, provides the member constant value equal to true, otherwise value is false.
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_default_constructible_v = is_default_constructible<T>::value;
template< class T > (since C++17)
inline constexpr bool is_trivially_default_constructible_v = is_trivially_default_constructible<T>::value;
template< class T > (since C++17)
inline constexpr bool is_nothrow_default_constructible_v = is_nothrow_default_constructible<T>::value;

Inherited from std::integral_constant

Member constants

value true if T is default-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_default_constructible : std::is_constructible<T> {};

  template< class T>
  struct is_trivially_default_constructible : std::is_trivially_constructible<T> {};

  template< class T>
  struct is_nothrow_default_constructible : std::is_nothrow_constructible<T> {};

Notes

In many implementations, is_nothrow_default_constructible also checks if the destructor throws because it is effectively noexcept(T()). Same applies to is_trivially_default_constructible, which, in these implementations, also requires that the destructor is trivial: GCC_bug_51452 LWG_issue_2116.
std::is_default_constructible<T> does not test that T x; would compile; it attempts direct-initialization with an empty argument list (see std::is_constructible). Thus, std::is_default_constructible_v<const int> and std::is_default_constructible_v<const int[10]> are true.

Example

// Run this code

  #include <iostream>
  #include <type_traits>

  struct Ex1 {
      std::string str; // member has a non-trivial default ctor
  };
  struct Ex2 {
      int n;
      Ex2() = default; // trivial and non-throwing
  };

  int main() {
      std::cout << std::boolalpha << "Ex1 is default-constructible? "
                << std::is_default_constructible<Ex1>::value << '\n'
                << "Ex1 is trivially default-constructible? "
                << std::is_trivially_default_constructible<Ex1>::value << '\n'
                << "Ex2 is trivially default-constructible? "
                << std::is_trivially_default_constructible<Ex2>::value << '\n'
                << "Ex2 is nothrow default-constructible? "
                << std::is_nothrow_default_constructible<Ex2>::value << '\n';
  }

Output:

  Ex1 is default-constructible? true
  Ex1 is trivially default-constructible? false
  Ex2 is trivially default-constructible? true
  Ex2 is nothrow default-constructible? true