NAME¶

std::common_iterator::operator++ - std::common_iterator::operator++

Synopsis¶

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: (3) (exposition
constexpr const std::iter_value_t<I>& operator*() const only*)
noexcept {
return keep_;
}

};

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 << ' ';
std::cout << '\n';
}

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 a proxy class is used to
in more situations keep the result
LWG 3546 C++20 initialization of the proxy object was situation and definition
sometimes ill-formed adjusted
LWG 3574 C++20 variant was fully constexpr (P2231R1) also made constexpr
but common_iterator was not
LWG 3595 C++20 functions of the proxy type lacked added
constexpr and noexcept

See also¶

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