NAME¶

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

Synopsis¶

Defined in header <algorithm>
Call signature
template< std::bidirectional_iterator I1, std::sentinel_for<I1> S1,

std::bidirectional_iterator I2 > (since
requires std::indirectly_movable<I1, I2> (1) C++20)
constexpr move_backward_result<I1, I2>

move_backward( I1 first, S1 last, I2 result );
template< ranges::bidirectional_range R, std::bidirectional_iterator I >

requires std::indirectly_movable<ranges::iterator_t<R>, I> (2) (since
constexpr move_backward_result<ranges::borrowed_iterator_t<R>, I> C++20)

move_backward( R&& r, I result );

Helper types¶

template< class I, class O > (3) (since
using move_backward_result = ranges::in_out_result<I, O>; 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 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.

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]: · · · ▄ ▅ ▁ ▂ ▃

See also¶

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