# std::counted_iterator<I>::operator++,+,+=,--,-,-=

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< cpp‎ | iterator‎ | counted iterator

constexpr counted_iterator& operator++(); | (1) | (since C++20)
constexpr decltype(auto) operator++( int ); | (2) | (since C++20)
constexpr counted_iterator operator++( int ) requires std::forward_iterator<I>; | (3) | (since C++20)
constexpr counted_iterator& operator--() requires std::bidirectional_iterator<I>; | (4) | (since C++20)
constexpr counted_iterator operator--( int ) requires std::bidirectional_iterator<I>; | (5) | (since C++20)
constexpr counted_iterator operator+( std::iter_difference_t<I> n ) const requires std::random_access_iterator<I>; | (6) | (since C++20)
constexpr counted_iterator& operator+=( std::iter_difference_t<I> n ) requires std::random_access_iterator<I>; | (7) | (since C++20)
constexpr counted_iterator operator-( std::iter_difference_t<I> n ) const requires std::random_access_iterator<I>; | (8) | (since C++20)
constexpr counted_iterator& operator-=( std::iter_difference_t<I> n ) requires std::random_access_iterator<I>; | (9) | (since C++20)

Increments or decrements the underlying iterator `current` and the distance to the end `length`.

The behavior of these functions is undefined if the `length` would be set to a minus value.

1) Pre-increments by one. Equivalent to ++current; --length; return *this;.
2) Post-increments by one. Equivalent to --length; try { return current++; } catch(...) { ++length; throw; }.
3) Post-increments by one. Equivalent to counted_iterator temp{*this}; ++*this; return temp;.
4) Pre-decrements by one. Equivalent to --current; ++length; return *this;.
5) Post-decrements by one. Equivalent to counted_iterator temp{*this}; --*this; return temp;.
6) Returns an iterator adaptor which is advanced by `n`. Equivalent to return counted_iterator(current + n, length - n);.
7) Advances the iterator adaptor by `n`. Equivalent to current += n; length -= n; return *this;.
8) Returns an iterator adaptor which is advanced by `-n`. Equivalent to return counted_iterator(current - n, length + n);.
9) Advances the iterator adaptor by `-n`. Equivalent to current -= n; length += n; return *this;.

### Parameters

n | - | the number of positions to increment or decrement the iterator adaptor

### Return value

1) *this.
2-3) A copy of *this that was made before the change.
4) *this.
5) A copy of *this that was made before the change.
6) An iterator adaptor which is advanced by n.
7) *this.
8) An iterator adaptor which is advanced by -n.
9) *this.

### Example

Run this code

    #include <cassert>
    #include <initializer_list>
    #include <iterator>

    int main()
    {
        const auto v = {1, 2, 3, 4, 5, 6};
        std::counted_iterator<std::initializer_list<int>::iterator> it1 {v.begin(), 5};

        ++it1;              assert(*it1 == 2 && it1.count() == 4);  // (1)
        auto it2 = it1++;   assert(*it2 == 2 && *it1 == 3);         // (3)
        --it1;              assert(*it1 == 2 && it1.count() == 4);  // (4)
        auto it3 = it1--;   assert(*it3 == 2 && *it1 == 1);         // (5)
        auto it4 = it1 + 3; assert(*it4 == 4 && it4.count() == 2);  // (6)
        auto it5 = it4 - 3; assert(*it5 == 1 && it5.count() == 5);  // (8)
        it1 += 3;           assert(*it1 == 4 && it1.count() == 2);  // (7)
        it1 -= 3;           assert(*it1 == 1 && it1.count() == 5);  // (9)
    }

### See also

operator+ (C++20) | advances the iterator (function template)
operator- (C++20) | computes the distance between two iterator adaptors (function template)

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