Apache C++ Standard Library Reference Guide

## Bidirectional Iterators

Library: Iterators

- Local Index
- Summary
- Description
- Key to Iterator Requirements
- Requirements for Bidirectional Iterators
- See Also
- Standards Conformance

### Local Index

No Entries

### Summary

An iterator that can both read and write and can traverse a container in both directions

### Description

> NOTE -- For a complete discussion of iterators, see the Iterators section of this reference.

Iterators are a generalization of pointers that allow a C++ program to uniformly interact with different data structures. Bidirectional iterators can move both forwards and backwards through a container, and have the ability to both read and write data. These iterators satisfy the requirements listed below.

### Key to Iterator Requirements

The following key pertains to the iterator descriptions listed below:

`a and b` | values of type `X`
`n` | value representing a `distance` between two iterators
`u, Distance, tmp and m` | identifiers
`r` | value of type `X&`
`t` | value of type `T`

### Requirements for Bidirectional Iterators

A bidirectional iterator must meet all the requirements listed below. Note that most of these requirements are also the requirements for forward iterators.

`X u` | u might have a singular value
`X()` | X() might be singular
`X(a)` | copy constructor,`a == X(a)`
`X u(a)` | copy constructor, `u == a`
`X u = a` | assignment, `u == a`
`r = a` | assignment, `r == a`
`a == b, a != b` | return value convertible to `bool`
`a->m` | equivalent to `(*a).m`
`*a` | return value convertible to `T&`
`++r` | returns `X&`
`r++` | return value convertible to const `X&`
`*r++` | returns `T&`
`--r` | returns `X&`
`r--` | return value convertible to const `X&`
`*r--` | returns `T&`

Like forward iterators, bidirectional iterators have the condition that `a == b` implies `*a== *b`.

There are no restrictions on the number of passes an algorithm may make through the structure.

### See Also

Containers, Iterators, Forward Iterators

### Standards Conformance

ISO/IEC 14882:1998 -- International Standard for Information Systems -- Programming Language C++, Section 24.1.4