Std::is_default_constructible,std::is_trivially_default_constructible, (3) Linux Manual Page

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

std::is_default_constructible,std::is_trivially_default_constructible, (3) Linux Manual Page