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

Defined in header `<algorithm>`
Call signature
template< std::bidirectional_iterator I1, std::sentinel_for<I1> S1, std::bidirectional_iterator I2 > requires std::indirectly_movable<I1, I2> constexpr move_backward_result<I1, I2> move_backward( I1 first, S1 last, I2 result );	(1)	(since C++20)
template< ranges::bidirectional_range R, std::bidirectional_iterator I > requires std::indirectly_movable<ranges::iterator_t<R>, I> constexpr move_backward_result<ranges::borrowed_iterator_t<R>, I> move_backward( R&& r, I result );	(2)	(since C++20)
Helper types
template< class I, class O > using move_backward_result = ranges::in_out_result<I, O>;	(3)	(since C++20)

1) Moves the elements in the range, defined by [first, last), to another range [result - N, result), 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 result 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 *(result - n) = ranges::iter_move(last - n) for each integer n, where 0 ≤ n < N.

The function-like entities described on this page are niebloids, that is:

Explicit template argument lists may not be specified when calling any of them.
None of them is visible to argument-dependent lookup.
When one of them is found by normal unqualified lookup for the name to the left of the function-call operator, it inhibits argument-dependent lookup.

In practice, they may be implemented as function objects, or with special compiler extensions.

Parameters

first	-	the beginning of the range of elements to move
last	-	the end of the range of elements to move
r	-	the range of the elements to move
result	-	the end of the destination range

Return value

{last, result - N}.

Complexity

1) Exactly N move assignments.

2) Exactly ranges::distance(r) move assignments.

Notes

When moving overlapping ranges, ranges::move is appropriate when moving to the left (beginning of the destination range is outside the source range) while ranges::move_backward is appropriate when moving to the right (end of the destination range is outside the source range).

Possible implementation

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 result ) const {
          auto i {last};
          for (; i != first; *--result = ranges::iter_move(--i));
          return {std::move(last), std::move(result)};
        }
 
  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 result ) const {
          return (*this)(ranges::begin(r), ranges::end(r), std::move(result));
        }
};
 
inline constexpr move_backward_fn move_backward{};

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]: · · · ▄ ▅ ▁ ▂ ▃

Compiler support
Freestanding and hosted
Language
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries

ranges::move (C++20)	moves a range of elements to a new location (niebloid)
ranges::copyranges::copy_if (C++20)(C++20)	copies a range of elements to a new location (niebloid)
ranges::copy_backward (C++20)	copies a range of elements in backwards order (niebloid)
move (C++11)	moves a range of elements to a new location (function template)
move (C++11)	obtains an rvalue reference (function template)