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

From cppreference.com
 
 
Iterator library
Iterator concepts
Iterator primitives
Algorithm concepts and utilities
Indirect callable concepts
Common algorithm requirements
(C++20)
(C++20)
(C++20)
Utilities
(C++20)
Iterator adaptors
Range access
(C++11)(C++14)
(C++14)(C++14)  
(C++11)(C++14)
(C++14)(C++14)  
(C++17)(C++20)
(C++17)
(C++17)
 
 
/* unspecified */ operator[]( difference_type n ) const;
(until C++17)
constexpr /* unspecified */ operator[]( difference_type n ) const;
(since C++17)

Returns a reference to the element at specified relative location.

Contents

[edit] Parameters

n - position relative to current location

[edit] Return value

A reference to the element at relative location, that is, base()[-n - 1].

[edit] Notes

The return type was changed by LWG386 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 of reverse_iterator::operator[] is declared to be reference. See also LWG2595.

[edit] 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:

3 2 1 0
3 2 1 0

[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 386 C++98 the return type was reference made unspecified

[edit] See also

accesses the pointed-to element
(public member function) [edit]