std::default_initializable (since C++20)

 

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Core language concepts same_as (C++20) derived_from (C++20) convertible_to (C++20) common_reference_with (C++20) common_with (C++20) integral (C++20) signed_integral (C++20) unsigned_integral (C++20) floating_point (C++20) swappableswappable_with (C++20)(C++20) destructible (C++20) constructible_from (C++20) default_initializable (C++20)   move_constructible (C++20) copy_constructible (C++20) assignable_from (C++20)

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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

• value-initialized (i.e., whether T() is well-formed);
• direct-list-initialized from an empty initializer list (i.e., whether T{} is well-formed); and
• default-initialized (i.e., whether 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{}; ::new T; };

References

• C++23 standard (ISO/IEC 14882:2024):

• 18.4.12 Concept default_initializable [concept.default.init]

• C++20 standard (ISO/IEC 14882:2020):

• 18.4.12 Concept default_initializable [concept.default.init]

See also

constructible_from (C++20)	specifies that a variable of the type can be constructed from or bound to a set of argument types (concept)
is_default_constructibleis_trivially_default_constructibleis_nothrow_default_constructible (C++11)(C++11)(C++11)	checks if a type has a default constructor (class template)