NAME¶

std::ranges::copy_backward,std::ranges::copy_backward_result - std::ranges::copy_backward,std::ranges::copy_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_copyable<I1, I2> (1) C++20)
constexpr copy_backward_result<I1, I2>

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

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

copy_backward( R&& r, I result );

Helper types¶

template< class I1, class I2 > (3) (since
using copy_backward_result = ranges::in_out_result<I1, I2>; C++20)

1) Copies the elements from the range, defined by [first, last), to another range
[result - N, result), where N = ranges::distance(first, last). The elements are
copied in reverse order (the last element is copied first), but their relative order
is preserved. The behavior is undefined if result is within (first, last]. In such a
case std::ranges::copy can 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 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 copy from
last - the end of the range of elements to copy from
r - the range of the elements to copy from
result - the end of the destination range

Return value¶

{last, result - N}

Complexity¶

Exactly N assignments.

Notes¶

When copying overlapping ranges, ranges::copy is appropriate when copying to the
left (beginning of the destination range is outside the source range) while
ranges::copy_backward is appropriate when copying to the right (end of the
destination range is outside the source range).

Possible implementation¶

struct copy_backward_fn
{
template<std::bidirectional_iterator I1, std::sentinel_for<I1> S1,
std::bidirectional_iterator I2>
requires std::indirectly_copyable<I1, I2>
constexpr ranges::copy_backward_result<I1, I2>
operator()(I1 first, S1 last, I2 result) const
{
I1 last1 {ranges::next(first, std::move(last))};
for (I1 i {last1}; i != first;)
*--result = *--i;
return {std::move(last1), std::move(result)};
}

template<ranges::bidirectional_range R, std::bidirectional_iterator I>
requires std::indirectly_copyable<ranges::iterator_t<R>, I>
constexpr ranges::copy_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 copy_backward_fn copy_backward{};

Example¶

// Run this code

#include <algorithm>
#include <iostream>
#include <ranges>
#include <string_view>
#include <vector>

void print(std::string_view rem, std::ranges::forward_range auto const& r)
{
for (std::cout << rem << ": "; auto const& elem : r)
std::cout << elem << ' ';
std::cout << '\n';
}

int main()
{
const auto src = {1, 2, 3, 4};
print("src", src);

std::vector<int> dst(src.size() + 2);
std::ranges::copy_backward(src, dst.end());
print("dst", dst);

std::ranges::fill(dst, 0);
const auto [in, out] =
std::ranges::copy_backward(src.begin(), src.end() - 2, dst.end());
print("dst", dst);

std::cout
<< "(in - src.begin) == " << std::distance(src.begin(), in) << '\n'
<< "(out - dst.begin) == " << std::distance(dst.begin(), out) << '\n';
}

Output:¶

src: 1 2 3 4
dst: 0 0 1 2 3 4
dst: 0 0 0 0 1 2
(in - src.begin) == 2
(out - dst.begin) == 4

See also¶

ranges::copy
ranges::copy_if copies a range of elements to a new location
(C++20) (niebloid)
(C++20)
ranges::copy_n copies a number of elements to a new location
(C++20) (niebloid)
ranges::remove_copy copies a range of elements omitting those that satisfy
ranges::remove_copy_if specific criteria
(C++20) (niebloid)
(C++20)
ranges::replace_copy copies a range, replacing elements satisfying specific
ranges::replace_copy_if criteria with another value
(C++20) (niebloid)
(C++20)
ranges::reverse_copy creates a copy of a range that is reversed
(C++20) (niebloid)
ranges::rotate_copy copies and rotate a range of elements
(C++20) (niebloid)
ranges::unique_copy creates a copy of some range of elements that contains no
(C++20) consecutive duplicates
(niebloid)
ranges::move moves a range of elements to a new location
(C++20) (niebloid)
ranges::move_backward moves a range of elements to a new location in backwards
(C++20) order
(niebloid)
copy_backward copies a range of elements in backwards order
(function template)