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std::common_iterator<I,S>::operator*,-> - cppreference.com

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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*/ {
    std::iter_value_t<I> keep_;
    constexpr proxy(std::iter_reference_t<I>&& x)
        : keep_(std::move(x)) {}
public:
    constexpr const std::iter_value_t<I>* operator->() const noexcept {
        return std::addressof(keep_);
    }
};
(4) (exposition only*)

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

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);.

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>> ).

If std::holds_alternative<I>(var ) is false, the behavior is undefined.

(until C++26)

If std::holds_alternative<I>(var ) is false:

(since C++26)

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.

Example

#include <complex>
#include <initializer_list>
#include <iostream>
#include <iterator>

using std::complex_literals::operator""i;

int main()
{
    const auto il = {1i, 3.14 + 2i, 3i, 4i, 5i};
    
    using CI = std::common_iterator<std::counted_iterator<decltype(il)::iterator>,
                                    std::default_sentinel_t>;
    
    CI ci{std::counted_iterator{std::next(begin(il), 1), std::ssize(il) - 1}};
    
    std::cout << *ci << ' ' << ci->real() << '\n';
}

Output:

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