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

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C++
Iterator library
Iterator concepts
Iterator primitives
Algorithm concepts and utilities
Indirect callable concepts
Common algorithm requirements
(C++20)
(C++20)
(C++20)
Utilities
(C++20)
Iterator adaptors
Range access
(C++11)(C++14)
(C++14)(C++14)
(C++11)(C++14)
(C++14)(C++14)
(C++17)(C++20)
(C++17)
(C++17)
std::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 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

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 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

 computes the distance between two iterator adaptors
(function template)
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