Home Original page

std::reverse_iterator<Iter>::operator[] - cppreference.com

From cppreference.com

/* 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

#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:

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