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

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

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.

  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