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std::common_iterator<I,S>::operator*,->

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Iterator library
Iterator concepts
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Algorithm concepts and utilities
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(C++20)
(C++20)
(C++20)
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(C++20)
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(C++11)    
(C++11)
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(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
Member functions
common_iterator::operator*common_iterator::operator->
(C++20)(C++20)
Non-member functions
(C++20)
(C++20)
(C++20)
(C++20)
Helper classes
 
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.

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>>

)
.

Contents

[edit] Parameters

(none)

[edit] 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.

[edit] 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:

(3.14,2) 3.14

[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
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

[edit] See also

constructs a new iterator adaptor
(public member function) [edit]