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std::default_initializable

From cppreference.com
< cpp‎ | concepts
Defined in header <concepts>
template < class T >

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

                                /* T t; is well-formed, see below */;
(since C++20)

The default_initializable concept checks whether variables of type T can be

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.

[edit] Possible implementation

template<class T>
concept default_initializable =
    std::constructible_from<T> &&
    requires { T{}; } &&
    requires { ::new (static_cast<void*>(nullptr)) T; };

[edit] See also

specifies that a variable of the type can be constructed from or bound to a set of argument types
(concept) [edit]
checks if a type has a default constructor
(class template) [edit]