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Bidirectional Iterator

Category: iterators Component type: concept


A Bidirectional Iterator is an iterator that can be both incremented and decremented. The requirement that a Bidirectional Iterator can be decremented is the only thing that distinguishes Bidirectional Iterators from Forward Iterators.

Refinement of

Forward Iterator

Associated types

The same as for Forward Iterator.


X A type that is a model of Bidirectional Iterator
T The value type of X
i, j Object of type X
t Object of type T


Valid expressions

In addition to the expressions defined in Forward Iterator, the following expressions must be valid.
Name Expression Type requirements Return type
Predecrement --i   X&
Postdecrement i--   X

Expression Semantics

Semantics of an expression is defined only where it is not defined in Forward Iterator.
Name Expression Precondition Semantics Postcondition
Predecrement --i i is dereferenceable or past-the-end. There exists a dereferenceable iterator j such that i == ++j. i is modified to point to the previous element. i is dereferenceable. &i = &--i. If i == j, then --i == --j. If j is dereferenceable and i == ++j, then --i == j.
Postdecrement i-- i is dereferenceable or past-the-end. There exists a dereferenceable iterator j such that i == ++j. Equivalent to
  X tmp = i; 
  return tmp; 

Complexity guarantees

The complexity of operations on bidirectional iterators is guaranteed to be amortized constant time.


Symmetry of increment and decrement If i is dereferenceable, then ++i; --i; is a null operation. Similarly, --i; ++i; is a null operation.



See also

Input Iterator, Output Iterator, Forward Iterator, Random Access Iterator, Iterator overview

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