constexpr decltype(auto) operator*();
(1) (since C++20)
constexpr decltype(auto) operator*() const
    requires /*dereferenceable*/<const I>;
(2) (since C++20)
constexpr auto operator->() const
    requires /* see description */;
(3) (since C++20)
Helper types
class /*proxy*/ {    // exposition only
    std::iter_value_t<I> keep_;
    constexpr proxy(std::iter_reference_t<I>&& x)
      : keep_(std::move(x)) {}
    constexpr const std::iter_value_t<I>* operator->() const noexcept {
        return std::addressof(keep_);
(4) (since C++20)

Returns pointer or reference to the current element, or a proxy holding it.

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).

1-2) Returns the result of dereferencing it.
3) Returns a pointer or underlying iterator to the current element, or a proxy holding it:
  • Equivalent to return it;, if I is a pointer type or if the expression it.operator->() is well-formed
  • Otherwise, equivalent to auto&& tmp = *it; return std::addressof(tmp);, if std::iter_reference_t<I> is a reference type,
  • Otherwise, equivalent to return proxy(*it);, where proxy is an exposition only class (4).
The expression in the requires-clause is equivalent to
std::indirectly_readable<const I> && (

requires(const I& i) { i.operator->(); } ||
std::is_reference_v<std::iter_reference_t<I>> ||
std::constructible_from<std::iter_value_t<I>, std::iter_reference_t<I>>




Return value

1-2) Reference to the current element, or prvalue temporary. Equivalent to *it.
3) Pointer or iterator to the current element or proxy holding it as described above.


#include <algorithm>
#include <complex>
#include <iostream>
#include <iterator>
#include <initializer_list>
using std::complex_literals::operator""i;
int main() {
    const auto il = {1i,3.14+2i,3i,4i,5i};
    using CI = std::common_iterator<
    CI ci { std::counted_iterator{
        std::next(begin(il), 1), ssize(il) - 1} };
    std::cout << *ci << ' ' << ci->real() << '\n';


(3.14,2) 3.14

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
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
LWG 3672 C++20 operator-> might return by reference in usual cases always returns by value

See also

constructs a new iterator adaptor
(public member function)

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