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std::iterator

Defined in header <iterator> 
template< 
    class Category,
    class T,
    class Distance = std::ptrdiff_t,
    class Pointer = T*,
    class Reference = T& 
> struct iterator;
 (deprecated in C++17)

std::iterator is the base class provided to simplify definitions of the required types for iterators.

Template parameters

Category - the category of the iterator. Must be one of iterator category tags.
T - the type of the values that can be obtained by dereferencing the iterator. This type should be void for output iterators.
Distance - a type that can be used to identify distance between iterators
Pointer - defines a pointer to the type iterated over (T)
Reference - defines a reference to the type iterated over (T)

Member types

Member type Definition
iterator_category Category
value_type T
difference_type Distance
pointer Pointer
reference Reference

Example

The following example shows how to implement an input iterator by inheriting from std::iterator

#include <algorithm>
#include <iostream>
 
template<long FROM, long TO>
class Range
{
public:
    // member typedefs provided through inheriting from std::iterator
    class iterator : public std::iterator<
                                std::input_iterator_tag, // iterator_category
                                long,                    // value_type
                                long,                    // difference_type
                                const long*,             // pointer
                                long                     // reference
                            > {
        long num = FROM;
    public:
        explicit iterator(long _num = 0) : num(_num) {}
        iterator& operator++() { num = TO >= FROM ? num + 1: num - 1; return *this; }
        iterator operator++(int) { iterator retval = *this; ++(*this); return retval; }
        bool operator==(iterator other) const { return num == other.num; }
        bool operator!=(iterator other) const { return !(*this == other); }
        reference operator*() const { return num; }
    };
    iterator begin() { return iterator(FROM); }
    iterator end() { return iterator(TO >= FROM? TO + 1 : TO - 1); }
};
 
int main()
{
    // std::find requires an input iterator
    auto range = Range<15, 25>();
    auto itr = std::find(range.begin(), range.end(), 18);
    std::cout << *itr << '\n'; // 18
 
    // Range::iterator also satisfies range-based for requirements
    for (long l : Range<3, 5>())
        std::cout << l << ' '; // 3 4 5
    std::cout << '\n';
}

Output:

18
3 4 5

See also

provides uniform interface to the properties of an iterator
(class template)
(C++20)
empty class types used to indicate iterator categories
(class)

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