NAME¶

std::reverse_iterator::operator[] - std::reverse_iterator::operator[]

Synopsis¶

/* 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.

Parameters¶

n - position relative to current location

Return value¶

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

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.

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