std::is_default_constructible,std::is_trivially_default_constructible,(3)

NAME
std::is_default_constructible,std::is_trivially_default_constructible, - std::is_default_constructible,std::is_trivially_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.
   The behavior of a program that adds specializations for any of the templates
   described on this page is undefined.
  Helper variable templates
   template< class T >
   inline constexpr bool is_default_constructible_v = (since C++17)
   is_default_constructible<T>::value;
   template< class T >
   inline constexpr bool is_trivially_default_constructible_v = (since C++17)
   is_trivially_default_constructible<T>::value;
   template< class T >
   inline constexpr bool is_nothrow_default_constructible_v = (since C++17)
   is_nothrow_default_constructible<T>::value; Inherited from std::integral_constant

Member constants
   value true if T is default-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_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

See also
   is_constructible
   is_trivially_constructible checks if a type has a constructor for specific
   is_nothrow_constructible arguments
   (C++11) (class template)
   (C++11)
   (C++11)
   is_copy_constructible
   is_trivially_copy_constructible
   is_nothrow_copy_constructible checks if a type has a copy constructor
   (C++11) (class template)
   (C++11)
   (C++11)
   is_move_constructible
   is_trivially_move_constructible checks if a type can be constructed from an rvalue
   is_nothrow_move_constructible reference
   (C++11) (class template)
   (C++11)
   (C++11)
   default_initializable specifies that an object of a type can be default
   (C++20) constructed
                        (concept)