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std::ranges::copy_n, std::ranges::copy_n_result

From cppreference.com
< 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::input_iterator I, std::weakly_incrementable O >

requires std::indirectly_copyable <I, O>
constexpr copy_n_result<I, O>

    copy_n( I first, std::iter_difference_t <I> n, O result );
(1) (since C++20)
Helper type
template< class I, class O >
using copy_n_result = ranges::in_out_result <I, O>;
(2) (since C++20)
1) Copies exactly n values from the range beginning at first to the range beginning at result by performing *(result + i) = *(first + i) for each integer in [0n). The behavior is undefined if result is within the range [firstfirst + n) (ranges::copy_backward might be used instead in this case).

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

[edit] Parameters

first - the beginning of the range of elements to copy from
n - number of the elements to copy
result - the beginning of the destination range

[edit] Return value

ranges::copy_n_result{first + n, result + n} or more formally, a value of type ranges::in_out_result that contains an input_iterator iterator equals to ranges::next (first, n) and a weakly_incrementable iterator equals to ranges::next (result, n).

[edit] Complexity

Exactly n assignments.

[edit] Notes

In practice, implementations of std::ranges::copy_n may avoid multiple assignments and use bulk copy functions such as std::memmove if the value type is TriviallyCopyable and the iterator types satisfy contiguous_iterator. Alternatively, such copy acceleration can be injected during an optimization phase of a compiler.

When copying overlapping ranges, std::ranges::copy_n is appropriate when copying to the left (beginning of the destination range is outside the source range) while std::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_n_fn
{
 template<std::input_iterator I, std::weakly_incrementable O>
 requires std::indirectly_copyable <I, O>
 constexpr ranges::copy_n_result<I, O>
 operator()(I first, std::iter_difference_t <I> n, O result) const
 {
 for (; n-- > 0; (void)++first, (void)++result)
 *result = *first;
 
 return {std::move(first), std::move(result)};
 }
};
 
inline constexpr copy_n_fn copy_n{};

[edit] Example

Run this code
#include <algorithm>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <string>
#include <string_view>
 
int main()
{
 const std::string_view in{"ABCDEFGH"};
 std::string out;
 
 std::ranges::copy_n(in.begin(), 4, std::back_inserter (out));
 std::cout << std::quoted (out) << '\n';
 
 out = "abcdefgh";
 const auto res{std::ranges::copy_n(in.begin(), 5, out.begin())};
 const auto i{std::distance (std::begin (in), res.in)};
 const auto j{std::distance (std::begin (out), res.out)};
 std::cout << "in[" << i << "] = '" << in[i] << "'\n"
 << "out[" << j << "] = '" << out[j] << "'\n";
}

Output:

"ABCD"
in[5] = 'F'
out[5] = 'f'

[edit] See also

copies a range of elements to a new location
(algorithm function object)[edit]
copies a range of elements in backwards order
(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]
(C++11)
copies a number of elements to a new location
(function template) [edit]
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