# std::common_iterator<I,S>::operator++

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< cpp‎ | iterator‎ | common iterator

constexpr common_iterator& operator++(); | (1) | (since C++20)
constexpr decltype(auto) operator++( int ); | (2) | (since C++20)
Helper types |  |
class /*postfix_proxy*/ { std::iter_value_t<I> keep_; constexpr postfix_proxy(std::iter_reference_t<I>&& x) : keep_(std::forward<std::iter_reference_t<I>>(x)) {} public: constexpr const std::iter_value_t<I>& operator*() const noexcept { return keep_; } }; | (3) | (exposition only*)

Increments the underlying iterator.

The behavior is undefined if the underlying std::variant member object `var` does not hold an object of type `I`, i.e. std::holds_alternative<I>(var) is equal to false.

Let `it` denote the iterator of type `I` held by `var`, that is std::get<I>(var).

1) Pre-increments by one. Equivalent to ++it; return *this;.
2) Post-increments by one:

- Equivalent to: auto tmp = *this; ++*this; return tmp;, if I models `forward_iterator`,
- Equivalent to: return it++;, if the variable definition auto&& ref = *it++; is well-formed, or either

  - std::indirectly_readable<I> or
  - std::constructible_from<std::iter_value_t<I>, std::iter_reference_t<I>> or
  - std::move_constructible<std::iter_value_t<I>>
    is false,

- Equivalent to: postfix_proxy p(**this); ++*this; return p; otherwise, where `postfix_proxy` is an exposition only helper type (3).

### Parameters

(none)

### Return value

1) *this.
2) A copy of *this that was made before the change, or a result of post-increment of the underlying iterator, or a proxy keeping the value of the current element, as described above.

### Example

Run this code

    #include <algorithm>
    #include <initializer_list>
    #include <iostream>
    #include <iterator>

    int main()
    {
        const auto il = {1, 2, 3, 4, 5, 6};

        using CI = std::common_iterator<
                       std::counted_iterator<std::initializer_list<int>::iterator>,
                       std::default_sentinel_t
                       >;

        CI first{std::counted_iterator{std::begin(il), std::ssize(il) - 2}};

        for (; first != std::default_sentinel; ++first)
            std::cout << *first << ' ';
    }

Output:

    1 2 3 4

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
P2259R1 | C++20 | post increment might discard its result in more situations | a proxy class is used to keep the result
LWG 3546 | C++20 | initialization of the proxy object was sometimes ill-formed | situation and definition adjusted
LWG 3574 | C++20 | `variant` was fully constexpr (P2231R1) but `common_iterator` was not | also made constexpr
LWG 3595 | C++20 | functions of the proxy type lacked constexpr and noexcept | added

### See also

operator- | computes the distance between two iterator adaptors (function template)

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