Standard Template Library Programmer's Guide

begin() and end() are amortized constant time.

size() is linear in the container's size. [10] max_size() and empty() are amortized constant time. If you are testing whether a container is empty, you should always write c.empty() instead of c.size() == 0 . The two expressions are equivalent, but the former may be much faster.

swap() is amortized constant time. [9]

• vector

[1] The fact that the lifetime of elements cannot exceed that of of their container may seem like a severe restriction. In fact, though, it is not. Note that pointers and iterators are objects; like any other objects, they may be stored in a container. The container, in that case, "owns" the pointers themselves, but not the objects that they point to.

[2] This expression must be a typedef , that is, a synonym for a type that already has some other name.

[3] This may either be a typedef for some other type, or else a unique type that is defined as a nested class within the class X .

[4] A container's iterator type and const iterator type may be the same: there is no guarantee that every container must have an associated mutable iterator type. For example, set and hash_set define iterator and const_iterator to be the same type.

[5] It is required that the reference type has the same semantics as an ordinary C++ reference, but it need not actually be an ordinary C++ reference. Some implementations, for example, might provide additional reference types to support non-standard memory models. Note, however, that "smart references" (user-defined reference types that provide additional functionality) are not a viable option. It is impossible for a user-defined type to have the same semantics as C++ references, because the C++ language does not support redefining the member access operator ( operator. ).

[6] As in the case of references [5], the pointer type must have the same semantics as C++ pointers but need not actually be a C++ pointer. "Smart pointers," however, unlike "smart references", are possible. This is because it is possible for user-defined types to define the dereference operator and the pointer member access operator, operator* and operator-> .

[7] The iterator type need only be an input iterator , which provides a very weak set of guarantees; in particular, all algorithms on input iterators must be "single pass". It follows that only a single iterator into a container may be active at any one time. This restriction is removed in Forward Container.

[8] In the case of a fixed-size container, size() == max_size() .

[9] For any Assignable type, swap can be defined in terms of assignment. This requires three assignments, each of which, for a container type, is linear in the container's size. In a sense, then, a.swap(b) is redundant. It exists solely for the sake of efficiency: for many containers, such as vector and list, it is possible to implement swap such that its run-time complexity is constant rather than linear. If this is possible for some container type X , then the template specialization swap(X&, X&) can simply be written in terms of X::swap(X&) . The implication of this is that X::swap(X&) should onlybe defined if there exists such a constant-time implementation. Not every container class X need have such a member function, but if the member function exists at all then it is guaranteed to be amortized constant time.

[10] For many containers, such as vector and deque , size is O(1 ). This satisfies the requirement that it be O(N ).

[11] Although [a.begin(), a.end()) must be a valid range, and must include every element in the container, the order in which the elements appear in that range is unspecified. If you iterate through a container twice, it is not guaranteed that the order will be the same both times. This restriction is removed in Forward Container.

The Iterator overview, Input Iterator, Sequence

Category: containers

Component type: concept

A Forward Container is a Container whose elements are arranged in a definite order: the ordering will not change spontaneously from iteration to iteration. The requirement of a definite ordering allows the definition of element-by-element equality (if the container's element type is Equality Comparable) and of lexicographical ordering (if the container's element type is LessThan Comparable).

Iterators into a Forward Container satisfy the forward iterator requirements; consequently, Forward Containers support multipass algorithms and allow multiple iterators into the same container to be active at the same time. Refinement of Container, EqualityComparable, LessThanComparable

No additional types beyond those defined in Container. However, the requirements for the iterator type are strengthened: the iterator type must be a model of Forward Iterator.

XA type that is a model of Forward Container

a, bObject of type X

TThe value type of X

In addition to the expressions defined in Container, EqualityComparable, and LessThanComparable, the following expressions must be valid.

Semantics of an expression is defined only where it is not defined in Container, EqualityComparable, or LessThanComparable, or where there is additional information.

The equality and inequality operations are linear in the container's size.