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Forward Container


Category: containers	Component type: concept

Description

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

Associated types

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.

Notation

`X`	A type that is a model of Forward Container
`a`, `b`	Object of type `X`
`T`	The value type of `X`

Definitions

Valid expressions

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

Name	Expression	Type requirements	Return type
Equality	`a == b`	`T` is EqualityComparable	Convertible to `bool`
Inequality	`a != b`	`T` is EqualityComparable	Convertible to `bool`
Less	`a < b`	`T` is LessThanComparable	Convertible to `bool`
Greater	`a > b`	`T` is LessThanComparable	Convertible to `bool`
Less or equal	`a <= b`	`T` is LessThanComparable	Convertible to `bool`
Greater or equal	`a >= b`	`T` is LessThanComparable	Convertible to `bool`

Expression semantics

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

Name	Expression	Precondition	Semantics	Postcondition
Equality	`a == b`		Returns `true` if `a.size() == b.size()` and if each element of `a` compares equal to the corresponding element of `b`. Otherwise returns `false`.
Less	`a < b`		Equivalent to `lexicographical_compare(a,b)`

Complexity guarantees

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

Invariants

Ordering

Two different iterations through a forward container will access its elements in the same order, providing that there have been no intervening mutative operations.