std::ranges...py_n_result(3)

💖 NAME 💖

okay so bestie, welcome to std::ranges::copy_n, std::ranges::copy_n_result — page std::ranges...py_n_result(3) from the C++ Standard Libary manual. Yes, the whole icon of this page is literally std::ranges::copy_n,std::ranges::copy_n_result, and we are going to get into it. 💅

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🌸 Synopsis 🌸

Defined in header <algorithm>
Call signature
template< std::input_iterator I, std::weakly_incrementable O >

requires std::indirectly_copyable<I, O>                        (1)
(since C++20)
constexpr copy_n_result<I, O>

    copy_n( I first, std::iter_difference_t<I> n, O result );
Helper type
template< class I, class O >                                   (2)
(since C++20)
using copy_n_result = ranges::in_out_result<I, O>;

1. copy_n behavior: 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 [0, n). The behavior is undefined if result is within the range [first, first + n) — and yeah, ranges::copy_backward might be used instead in this case.

🚨 bestie, that undefined behavior is real. Do not play with overlapping ranges like that unless you know exactly what you're doing. ranges::copy_backward is the backup dancer for a reason.

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. ✨

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🎀 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.

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🦋 Return value 🦋

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

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✨ Complexity ✨

Exactly n assignments. no cap, that's the whole count. 💖

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💅 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). 🫶

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🧚 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 (std::iter_difference_t<I> i {}; i != n; ++i, ++first, ++result)
            *result = *first;
        return {std::move(first), std::move(result)};
    }
};

inline constexpr copy_n_fn copy_n {};

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🍓 Example 🍓

// Run this code — go off:

#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());
    std::cout
        << "*(res.in): '" << *(res.in) << "', distance: "
        << std::distance(std::begin(in), res.in) << '\n'
        << "*(res.out): '" << *(res.out) << "', distance: "
        << std::distance(std::begin(out), res.out) << '\n';
}

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⭐ Output ⭐

"ABCD"
*(res.in): 'F', distance: 5
*(res.out): 'f', distance: 5

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🌈 See also 🌈

• ranges::copy — copies a range of elements to a new location (C++20) (niebloid)
• ranges::copy_if — copies a range of elements to a new location (C++20) (niebloid)
• ranges::copy_backward — copies a range of elements in backwards order (C++20) (niebloid)
• ranges::remove_copy — copies a range of elements omitting those that satisfy specific criteria (C++20) (niebloid)
• ranges::remove_copy_if — copies a range of elements omitting those that satisfy specific criteria (C++20) (niebloid)
• ranges::replace_copy — copies a range, replacing elements satisfying specific criteria with another value (C++20) (niebloid)
• ranges::replace_copy_if — copies a range, replacing elements satisfying specific criteria with another value (C++20) (niebloid)
• 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 consecutive duplicates (C++20) (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 order (C++20) (niebloid)
• copy_n — copies a number of elements to a new location (C++11) (function template)

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source/version: http://cppreference.com · date: 2024.06.10 · page: std::ranges...py_n_result(3) — mwah, bestie 💋💖