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std::ranges::copy_backward, std::ranges::copy_backward_result

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< cpp‎ | algorithm‎ | ranges
 
 
Algorithm library
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
(C++11)
(C++11)
(C++17)

 
Constrained algorithms
All names in this menu belong to namespace std::ranges
       
       
    
     
         
       
       
(C++23)
(C++23)  
(C++23)
(C++23)  
(C++23)            
 
Defined in header <algorithm>
Call signature
template< std::bidirectional_iterator I1, std::sentinel_for <I1> S1,

          std::bidirectional_iterator I2 >
requires std::indirectly_copyable <I1, I2>
constexpr copy_backward_result<I1, I2>

    copy_backward( I1 first, S1 last, I2 d_last );
(1) (since C++20)
template< ranges::bidirectional_range R, std::bidirectional_iterator I >

requires std::indirectly_copyable <ranges::iterator_t <R>, I>
constexpr copy_backward_result<ranges::borrowed_iterator_t <R>, I>

    copy_backward( R&& r, I d_last );
(2) (since C++20)
Helper types
template< class I1, class I2 >
using copy_backward_result = ranges::in_out_result <I1, I2>;
(3) (since C++20)
1) Copies the elements from the range, defined by [firstlast), to another range [d_last - Nd_last), 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 d_last 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 algorithm function objects (informally known as niebloids), that is:

[edit] Parameters

first, last - the iterator-sentinel pair defining the range of elements to copy from
r - the range of the elements to copy from
d_last - the end of the destination range

[edit] Return value

{last, d_last - N}

[edit] Complexity

Exactly N assignments.

[edit] 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).

[edit] 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 d_last) const
 {
 I1 last1 {ranges::next (first, std::move(last))};
 for (I1 i {last1}; i != first;)
 *--d_last = *--i;
 return {std::move(last1), std::move(d_last)};
 }
 
 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 d_last) const
 {
 return (*this)(ranges::begin (r), ranges::end (r), std::move(d_last));
 }
};
 
inline constexpr copy_backward_fn copy_backward{};

[edit] 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

[edit] See also

copies a range of elements to a new location
(algorithm function object)[edit]
copies a number of elements to a new location
(algorithm function object)[edit]
copies a range of elements omitting those that satisfy specific criteria
(algorithm function object)[edit]
copies a range, replacing elements satisfying specific criteria with another value
(algorithm function object)[edit]
creates a copy of a range that is reversed
(algorithm function object)[edit]
copies and rotate a range of elements
(algorithm function object)[edit]
creates a copy of some range of elements that contains no consecutive duplicates
(algorithm function object)[edit]
moves a range of elements to a new location
(algorithm function object)[edit]
moves a range of elements to a new location in backwards order
(algorithm function object)[edit]
copies a range of elements in backwards order
(function template) [edit]
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