C++ named requirements: DefaultConstructible

 

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Technical specifications

Symbols index

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C++ named requirements

Basic
DefaultConstructible CopyAssignable	CopyConstructible MoveAssignable (C++11)	MoveConstructible (C++11) Destructible
Type properties
ScalarType TriviallyCopyable (C++11)	StandardLayoutType (C++11) PODType	TrivialType (C++11) ImplicitLifetimeType
Library-Wide
EqualityComparable LessThanComparable Swappable ValueSwappable (C++11)	NullablePointer (C++11) Hash (C++11) Allocator FunctionObject	Callable Predicate BinaryPredicate Compare
Container
Container SequenceContainer ContiguousContainer (C++17)	ReversibleContainer AssociativeContainer AllocatorAwareContainer	UnorderedAssociativeContainer (C++11)
Container Elements
DefaultInsertable (C++11) CopyInsertable (C++11)	MoveInsertable (C++11) EmplaceConstructible (C++11)	Erasable (C++11)
Iterator
LegacyIterator LegacyOutputIterator LegacyInputIterator	LegacyForwardIterator LegacyBidirectionalIterator LegacyRandomAccessIterator	LegacyContiguousIterator (C++17) ConstexprIterator (C++20)
Stream I/O
FormattedInputFunction UnformattedInputFunction	FormattedOutputFunction	UnformattedOutputFunction
Formatters
BasicFormatter (C++20)	Formatter (C++20)
Random Numbers
SeedSequence (C++11) RandomNumberEngine (C++11)	RandomNumberDistribution (C++11) UniformRandomBitGenerator (C++11)	RandomNumberEngineAdaptor (C++11)
Concurrency
BasicLockable (C++11) Lockable (C++11) TimedLockable (C++11)	SharedLockable (C++14) SharedTimedLockable (C++14) Mutex (C++11)	TimedMutex (C++11) SharedMutex (C++17) SharedTimedMutex (C++14)
Ranges
RangeAdaptorObject (C++20)	RangeAdaptorClosureObject (C++20)
Other
CharTraits RegexTraits (C++11) BitmaskType LiteralType (C++11)	NumericType UnaryTypeTrait (C++11) BinaryTypeTrait (C++11) TransformationTrait (C++11)	Clock (C++11) TrivialClock (C++11)

 

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

Requirements

The type T satisfies DefaultConstructible if

Given

• u, an arbitrary identifier.

The following expressions must be valid and have their specified effects.

Expression	Post-conditions
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

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)
default_initializable (C++20)	specifies that an object of a type can be default constructed (concept)