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Category: iterators Component type: type


Forward_iterator is an iterator base class: it is intended that an iterator that is a model of ForwardIterator, and whose value type and distance type are T and Distance, may be defined by inheriting from forward_iterator<T, Distance> [1]. Forward_iterator is entirely empty: it has no member functions, member variables, or nested types. It exists solely to simplify the definition of the functions iterator_category, distance_type, and value_type.


class my_forward_iterator : public forward_iterator<double> 

This declares my_forward_iterator to be a ForwardIterator whose value type is double and whose distance type is ptrdiff_t. If Iter is an object of class my_forward_iterator, then iterator_category(Iter) will return forward_iterator_tag(), value_type(Iter) will return (double*) 0, and distance_type(Iter) will return (ptrdiff_t*) 0.


Defined in the standard header iterator, and in the nonstandard backward-compatibility header iterator.h. This class is no longer part of the C++ standard, although it was present in early drafts of the standard. It is retained in this implementation for backward compatibility.

Template parameters

Parameter Description Default
T The iterator's value type  
Distance The iterator's distance type ptrdiff_t

Model of


Public base classes


Type requirements

The distance type must be a signed integral type.

Public base classes




New Members



[1] It is not required that a ForwardIterator inherit from the base forward_iterator. It is, however, required that the functions iterator_category, distance_type, and value_type be defined for every ForwardIterator. (Or, if you are using the iterator_traits mechanism, that iterator_traits is properly specialized for every ForwardIterator.) Since those functions are defined for the base forward_iterator, the easiest way to ensure that are defined for a new type is to derive that class from forward_iterator and rely on the derived-to-base standard conversion of function arguments.

See also

The iterator_tags overview, iterator_traits, iterator_category, value_type, distance_type, input_iterator, output_iterator, bidirectional_iterator, random_access_iterator

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