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.


  • T, an element type;
  • C, a Container type containing elements of type T;
  • a and b, objects of type C;
  • rv, a prvalue expression of type C.


Name Type Requirements
value_type T CopyConstructible (until C++11)Erasable (since C++11)
reference T&
const_reference const T&
iterator Iterator whose value type is T LegacyForwardIterator
convertible to const_iterator
const_iterator Constant iterator whose value type is 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

Member functions and operators

Expression Return type Semantics Conditions Complexity
C() C Creates an empty container Post: C().empty()
== true
C(a) C Creates a copy of a Pre: T must be CopyInsertable
Post: a == C(a)
(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
(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
(since C++11)
const_iterator const_cast<const C&>(a).begin() Constant
(since C++11)
const_iterator const_cast<const C&>(a).end() Constant
a == b Convertible to bool
a.size() == b.size() &&


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. (since C++11) 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

C (Container).

T (Type).

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 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

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