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

 

C++

Compiler support

Freestanding and hosted

Language

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Iterator library

Iterator concepts
indirectly_readable (C++20) indirectly_writable (C++20) weakly_incrementable (C++20) incrementable (C++20) input_or_output_iterator (C++20) sentinel_for (C++20) sized_sentinel_for (C++20)	input_iterator (C++20) output_iterator (C++20) forward_iterator (C++20) bidirectional_iterator (C++20) random_access_iterator (C++20) contiguous_iterator (C++20)
Iterator primitives
iterator_traits input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tagcontiguous_iterator_tag (C++20) iterator (deprecated in C++17)	incrementable_traits (C++20) indirectly_readable_traits (C++20) iter_value_titer_difference_titer_reference_titer_const_reference_titer_rvalue_reference_titer_common_reference_t (C++20)(C++20)(C++20)(C++23)(C++20)(C++20)
Algorithm concepts and utilities
Indirect callable concepts
indirectly_unary_invocableindirectly_regular_unary_invocable (C++20)(C++20)   indirect_unary_predicate (C++20)	indirect_binary_predicate (C++20) indirect_equivalence_relation (C++20) indirect_strict_weak_order (C++20)
Common algorithm requirements
indirectly_movable (C++20) indirectly_movable_storable (C++20) indirectly_copyable (C++20) indirectly_copyable_storable (C++20)	indirectly_swappable (C++20) indirectly_comparable (C++20) permutable (C++20) mergeable (C++20) sortable (C++20)
Utilities
indirect_result_t (C++20)	projected (C++20)
Iterator adaptors
reverse_iterator move_iterator (C++11) move_sentinel (C++20) common_iterator (C++20) counted_iterator (C++20) basic_const_iterator (C++23) const_iterator (C++23) const_sentinel (C++23) back_insert_iterator front_insert_iterator insert_iterator	make_reverse_iterator (C++14) make_move_iterator (C++11) default_sentinel_tdefault_sentinel (C++20)(C++20) unreachable_sentinel_tunreachable_sentinel (C++20)(C++20) make_const_iterator (C++23) make_const_sentinel (C++23) back_inserter front_inserter inserter
Stream iterators
istream_iterator ostream_iterator	istreambuf_iterator ostreambuf_iterator
Iterator customization points
ranges::iter_move (C++20)
ranges::iter_swap (C++20)
Iterator operations
advance distance prev (C++11) next (C++11)	ranges::advance (C++20) ranges::distance (C++20) ranges::prev (C++20) ranges::next (C++20)
Range access
begincbegin (C++11)(C++14) endcend (C++11)(C++14) sizessize (C++17)(C++20)	rbegincrbegin (C++14)(C++14) rendcrend (C++14)(C++14) empty (C++17) data (C++17)

 

std::common_iterator

Member functions

common_iterator::common_iterator (C++20)

common_iterator::operator= (C++20)

common_iterator::operator*common_iterator::operator-> (C++20)(C++20)

common_iterator::operator++common_iterator::operator++(int) (C++20)(C++20)

Non-member functions

operator== (C++20)

operator- (C++20)

iter_move (C++20)

iter_swap (C++20)

Helper classes

incrementable_traits (C++20)

iterator_traits (C++20)

 

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::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 <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
                   >;
 
    CI first { std::counted_iterator{ std::begin(il), std::ssize(il) - 2 } };
    CI last { std::default_sentinel };
 
    while (first != last)
    {
        std::cout << *first;
        std::cout << (++first != last ? ", " : "\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

operator-

computes the distance between two iterator adaptors (function template)