C++ named requirements: DefaultConstructible

Specifies that an instance of the type can be default constructed.

Requirements

The type T satisfies DefaultConstructible if all following statements and expressions are valid and have their specified effects:

Expression/Statement	Postcondition
T u;	The object u is default-initialized.
T u{};	The object u is value-initialized or aggregate-initialized.
T() T{}	A temporary object of type T is value-initialized or aggregate-initialized.

Notes

For objects of non-aggregate class type, a public default constructor must be defined (either user-defined or implicitly defined) to satisfy DefaultConstructible.

Non-const objects of non-class object type are always DefaultConstructible.

Const non-class types are not DefaultConstructible.

Const aggregate types are not DefaultConstructible if any of their members is an object of non-class type.

Non-object types (function types, reference types, and the (possibly cv-qualified) type void) as well as the const non-object types are never DefaultConstructible.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 724	C++98	the requirements of DefaultConstructible were missing	added
LWG 2170	C++98	initialzing an object of a DefaultConstructible type with an empty initializer could only result in value-initialization	can also lead to aggregate-initialization

See also

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)
is_default_constructible_typeis_trivially_default_constructible_typeis_nothrow_default_constructible_type (C++26)(C++26)(C++26)	checks if reflected type has a default constructor (function)
default_initializable (C++20)	specifies that an object of a type can be default constructed (concept)