C++ named requirements: Container

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.


  • C container type;
  • T element type;
  • a, b objects of type C.
  • rv a prvalue expression of type C.


name type notes
value_type T Erasable
reference T&
const_reference const T&
iterator iterator pointing to T LegacyForwardIterator
convertible to const_iterator
const_iterator constant iterator pointing to T LegacyForwardIterator
difference_type signed integer must be the same as iterator_traits::difference_type for iterator and const_iterator
size_type unsigned integer large enough to represent all positive values of difference_type

Methods 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)
C(rv)(since C++11) C moves rv Post: equal to the value rv had before this construction Constant[1]
a = b C& destroys or copy-assigns all elements of a from elements of b Post: a == b Linear
a = rv 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
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[2] 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[1][3]
swap(a, b) void a.swap(b) Constant[1]
a.size() size_type std::distance(a.begin(), a.end()) Constant[3]
a.max_size() size_type b.size() where b is the largest possible container Constant[3]
a.empty() convertible to bool a.begin() == a.end() Constant
  1. Linear for std::array
  2. always linear for std::forward_list
  3. (until C++11) Not strictly constant


  • 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.

Container data races

see container thread safety.

Other requirements


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 2263 C++11 resolution of LWG179 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 unspecifed

See also

C++ documentation for Containers library

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