C++ named requirements: Container

 

C++

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Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

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General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

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Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

C++ named requirements

Basic DefaultConstructible MoveConstructible (C++11) CopyConstructible CopyAssignable MoveAssignable (C++11) Destructible Type properties ScalarType PODType TriviallyCopyable (C++11) TrivialType (C++11) StandardLayoutType (C++11)   ImplicitLifetimeType Library-wide BooleanTestable EqualityComparable LessThanComparable Swappable ValueSwappable (C++11) NullablePointer (C++11) Hash (C++11) Allocator FunctionObject Callable Predicate BinaryPredicate Compare

Container Container ReversibleContainer AllocatorAwareContainer SequenceContainer ContiguousContainer (C++17) AssociativeContainer UnorderedAssociativeContainer (C++11)   Container element DefaultInsertable (C++11) CopyInsertable (C++11) MoveInsertable (C++11) EmplaceConstructible (C++11) Erasable (C++11) Iterator LegacyIterator LegacyInputIterator LegacyOutputIterator 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) Multidimensional View LayoutMapping (C++23) LayoutMappingPolicy (C++23) AccessorPolicy (C++23) 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)

 

A Container is an object used to store other objects and taking care of the management of the memory used by the objects it contains.

Requirements

Legend

• T, an element type;
• C, a Container type containing elements of type T;
• a and b, objects of type C;
• i and j, values of (possibly const) type C::iterator;
• rv, a prvalue expression of type C.

Member types

Name	Type	Requirements
C::value_type	T	CopyConstructible(until C++11)Erasable(since C++11)
C::reference	T&
C::const_reference	const T&
C::iterator	Iterator whose value type is T	LegacyForwardIterator convertible to const_iterator
C::const_iterator	Constant iterator whose value type is T	LegacyForwardIterator
C::difference_type	Signed integer	Must be the same as iterator_traits::difference_type for iterator and const_iterator
C::size_type	Unsigned integer	Large enough to represent all positive values of difference_type

Member functions and operators

Expression	Return type	Semantics	Conditions	Complexity
C()	C	Creates an empty container	Post: C().empty() == true	Constant
C(a)	C	Creates a copy of a	Pre: T must be CopyInsertable Post: a == C(a)	Linear
C(rv) (since C++11)	C	Moves rv	Post: equal to the value rv had before this construction	Constant
a = b	C&	Destroys or copy-assigns all elements of a from elements of b	Post: a == b	Linear
a = rv (since C++11)	C&	Destroys or move-assigns all elements of a from elements of rv	Post: if a and rv do not refer the same object, a is equal to the value rv had before this assignment	Linear
a.~C()	void	Destroys all elements of a and frees all memory		Linear
a.begin()	(const_)iterator	Iterator to the first element of a		Constant
a.end()	(const_)iterator	Iterator to one past the last element of a		Constant
a.cbegin() (since C++11)	const_iterator	const_cast<const C&>(a).begin()		Constant
a.cend() (since C++11)	const_iterator	const_cast<const C&>(a).end()		Constant
i <=> j (since C++20)	strong_ordering	Three-way comparison of container iterators	C::​iterator meets the random access iterator requirements	Constant
a == b	Convertible to bool	a.size() == b.size() &&     std::equal(a.begin(),         a.end(), b.begin()) (until C++14) std::equal(a.begin(), a.end(),     b.begin(), b.end()) (since C++14)	Pre: T must be EqualityComparable	Constant if a.size() != b.size(), linear otherwise
a != b	convertible to bool	!(a == b)		Linear
a.swap(b)	void	Exchanges the values of a and b		Constant
swap(a, b)	void	a.swap(b)		Constant
a.size()	size_type	std::distance(a.begin(), a.end())		Constant
a.max_size()	size_type	b.size() where b is the largest possible container		Constant
a.empty()	Convertible to bool	a.begin() == a.end()		Constant
Notes

Given

• i and j, objects of a container's iterator type,

in the expressions i == j, i != j, i < j, i <= j, i >= j, i > j, i - j, either or both may be replaced by an object of the container's const_iterator type referring to the same element with no change in semantics.

Optional container requirements (since C++20)

The following operations are provided only for some types of containers.

If the iterators passed to std::lexicographical_compare_three_way meet the ConstexprIterator then the operations described below are implemented by constexpr functions.

Category	Description
Expression	a <=> b
Returns	std::lexicographical_compare_three_way(a.begin(), a.end(),                                        b.begin(), b.end(),                                        /*synth-three-way*/)
Result	/*synth-three-way-result*/<C::value_type>
Preconditions	Either T models three_way_comparable, or < is defined for values of (possibly const) type T and < is a total ordering relationship.
Complexity	Linear

Container data races

See container thread safety.

Other requirements

C (Container)

• DefaultConstructible
• CopyConstructible
• EqualityComparable
• Swappable

T (Type)

• CopyInsertable
• EqualityComparable
• Destructible

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 179	C++98	iterator and const_iterator types might be incomparable	required to be comparable
LWG 276	C++98	T was required to be CopyAssignable	T is required to be CopyConstructible
LWG 322	C++98	the value types of iterator and const_iterator were not specified	specified as T
LWG 774	C++98	there was no requirement on swap(a, b)	added
LWG 883	C++98	a.swap(b) was defined as swap(a, b), resulted in circular definition	defined as exchanging the values of a and b
LWG 1319	C++98	iterator and const_iterator might not have multipass guarantee	they are required to satisfy the requirements of LegacyForwardIterator
LWG 2263	C++11	the resolution of LWG issue 179 was accidentally dropped in C++11	restored
LWG 2839	C++11	self move assignment of standard containers was not allowed	allowed but the result is unspecified

See also

C++ documentation for Containers library