std::ranges::move_backward, std::ranges::move_backward_result

1) Moves the elements in the range, defined by [first, last), to another range [d_last - N, d_last), where N = ranges::distance (first, last). The elements are moved in reverse order (the last element is moved first), but their relative order is preserved. The behavior is undefined if d_last is within (first, last]. In such a case, ranges::move may be used instead.

2) Same as (1), but uses r as the source range, as if using ranges::begin (r) as first, and ranges::end (r) as last.

The elements in the moved-from range will still contain valid values of the appropriate type, but not necessarily the same values as before the move, as if using *(d_last - n) = ranges::iter_move (last - n) for each integer n, where 0 ≤ n < N.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

Explicit template argument lists cannot be specified when calling any of them.
None of them are visible to argument-dependent lookup.
When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

struct move_backward_fn
{
 template<std::bidirectional_iterator I1, std::sentinel_for <I1> S1,
 std::bidirectional_iterator I2>
 requires std::indirectly_movable <I1, I2>
 constexpr ranges::move_backward_result<I1, I2>
 operator()(I1 first, S1 last, I2 d_last) const
 {
 auto i {last};
 for (; i != first; *--d_last = ranges::iter_move (--i))
 {}
 return {std::move(last), std::move(d_last)};
 }
 
 template<ranges::bidirectional_range R, std::bidirectional_iterator I>
 requires std::indirectly_movable <ranges::iterator_t <R>, I>
 constexpr ranges::move_backward_result<ranges::borrowed_iterator_t <R>, I>
 operator()(R&& r, I d_last) const
 {
 return (*this)(ranges::begin (r), ranges::end (r), std::move(d_last));
 }
};
 
inline constexpr move_backward_fn move_backward {};

[edit] Example

Run this code

#include <algorithm>
#include <iostream>
#include <string>
#include <string_view>
#include <vector>
 
using Vec = std::vector <std::string >;
 
void print(std::string_view rem, Vec const& vec)
{
 std::cout << rem << "[" << vec.size() << "]: ";
 for (const std::string & s : vec)
 std::cout << (s.size() ? s : std::string {"·"}) << ' ';
 std::cout << '\n';
}
 
int main()
{
 Vec a{"▁", "▂", "▃", "▄", "▅", "▆", "▇", "█"};
 Vec b(a.size());
 
 print("Before move:\n" "a", a);
 print("b", b);
 
 std::ranges::move_backward(a, b.end());
 
 print("\n" "Move a >> b:\n" "a", a);
 print("b", b);
 
 std::ranges::move_backward(b.begin(), b.end(), a.end());
 print("\n" "Move b >> a:\n" "a", a);
 print("b", b);
 
 std::ranges::move_backward(a.begin(), a.begin()+3, a.end());
 print("\n" "Overlapping move a[0, 3) >> a[5, 8):\n" "a", a);
}

Possible output:

Before move:
a[8]: ▁ ▂ ▃ ▄ ▅ ▆ ▇ █
b[8]: · · · · · · · ·
 
Move a >> b:
a[8]: · · · · · · · ·
b[8]: ▁ ▂ ▃ ▄ ▅ ▆ ▇ █
 
Move b >> a:
a[8]: ▁ ▂ ▃ ▄ ▅ ▆ ▇ █
b[8]: · · · · · · · ·
 
Overlapping move a[0, 3) >> a[5, 8):
a[8]: · · · ▄ ▅ ▁ ▂ ▃