std::common_iterator<I,S>::operator++
From cppreference.com
< 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_; |
(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).
Contents |
[edit] Parameters
(none)
[edit] 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.
[edit] 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
[edit] 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 |
[edit] See also
computes the distance between two iterator adaptors (function template) |