# std::reverse_iterator<Iter>::operator[]

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< cpp‎ | iterator‎ | reverse iterator

/* unspecified */ operator[]( difference_type n ) const; |  | (constexpr since C++17)

Returns a reference to the element at specified relative location.

### Parameters

n | - | position relative to current location

### Return value

`current`[-n - 1]

### Notes

The return type was changed by LWG issue 386 to be unspecified because the return type of the underlying iterator's operator[] was also unspecified at the time.

However, as of N3066, the return type of a LegacyRandomAccessIterator's operator[] is required to be convertible to reference. In all common implementations, the return type is declared to be reference. See also LWG issue 2595.

### Example

Run this code

    #include <array>
    #include <cstddef>
    #include <iostream>
    #include <iterator>
    #include <list>
    #include <vector>

    int main()
    {
        int a[]{0, 1, 2, 3};
        std::reverse_iterator<int*> iter1{std::rbegin(a)};
        for (std::size_t i{}; i != std::size(a); ++i)
            std::cout << iter1[i] << ' '; // decltype(iter1[i]) is int&
        std::cout << '\n';

        std::vector v{0, 1, 2, 3};
        std::reverse_iterator<std::vector<int>::iterator> iter2{std::rbegin(v)};
        for (std::size_t i{}; i != std::size(v); ++i)
            std::cout << iter2[i] << ' '; // decltype(iter2[i]) is int&
        std::cout << '\n';

        // constexpr context
        constexpr static std::array<int, 4> z{0, 1, 2, 3};
        constexpr std::reverse_iterator<decltype(z)::const_iterator> iter3{std::crbegin(z)};
        static_assert(iter3[1] == 2);

        std::list li{0, 1, 2, 3};
        std::reverse_iterator<std::list<int>::iterator> iter4{std::rbegin(li)};
        *iter4 = 42;   // OK
    //  iter4[0] = 13; // Compilation error: the underlying iterator
                       // does not model the random access iterator
    }

Output:

    3 2 1 0
    3 2 1 0

### 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 386 | C++98 | the return type was `reference` | made unspecified

### See also

operator*operator-> | accesses the pointed-to element (public member function)

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