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

From cppreference.com
 
 
Iterator library
Iterator concepts
Iterator primitives
Algorithm concepts and utilities
Indirect callable concepts
Common algorithm requirements
Utilities
(C++20)
Iterator adaptors
Stream iterators
Iterator customization points
Iterator operations
(C++11)
(C++11)
Range access
(C++11)(C++14)
(C++11)(C++14)
(C++17)(C++20)
(C++14)(C++14)
(C++14)(C++14)
(C++17)
(C++17)
 
std::common_iterator
Member functions
common_iterator::operator++common_iterator::operator++(int)
(C++20)(C++20)
Non-member functions
(C++20)
(C++20)
(C++20)
(C++20)
Helper classes
 
constexpr common_iterator& operator++();
(1) (since C++20)
constexpr decltype(auto) operator++( int );
(2) (since C++20)
Helper types
class /*postfix_proxy*/ {   // exposition only

    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) (since C++20)

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::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

#include <algorithm>
#include <iostream>
#include <iterator>
#include <initializer_list>
 
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
                   >;
 
    for ( CI first { std::counted_iterator{ std::begin(il), std::ssize(il) - 2 } },
             last { std::default_sentinel }; first != last; ) {
        std::cout << *first;
        std::cout << (++first != last ? ", " : "\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) [edit]