std::reverse_iterator<Iter>::operator[]
Min standard notice:
Returns a reference to the element at specified relative location.
# Declarations
/* unspecified */ operator[]( difference_type n ) const;
(constexpr since C++17)
# 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
}
# Defect reports
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 386 | C++98 | the return type was reference | made unspecified |