std::default_initializable

template< class T >
concept default_initializable = std::constructible_from<T> && requires { T{}; } && 
                                /* T t; is well-formed, see below */;

The default_initializable concept checks whether variables of type T can be

• value-initialized (T() is well-formed);
• direct-list-initialized from an empty initializer list (T{} is well-formed); and
• default-initialized (T t; is well-formed).

Access checking is performed as if in a context unrelated to T. Only the validity of the immediate context of the variable initialization is considered.

Possible implementation

template<class T>
concept default_initializable =
    std::constructible_from<T> &&
    requires { T{}; } &&
    requires { ::new T; };

See also