std::ranges::fill_n(3)

std::ranges::fill_n(3)

okay bestie, welcome to the std::ranges::fill_n page from the C++ Standard Libary manual — yes, that’s literally how the header spells “Libary,” not me 💀. It’s giving C++20/26 algorithm energy and I’m obsessed.

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🎀 NAME 🎀

std::ranges::fill_n — std::ranges::fill_n (the name is the name, iconic 💅)

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✨ Synopsis ✨

Defined in header <algorithm>.

Call signature:

template< class T, std::output_iterator<const T&> O >
constexpr O fill_n( O first, std::iter_difference_t<O> n, const T& value );

(since C++20) (until C++26)

template< class O, class T = std::iter_value_t<O> >

requires std::output_iterator<O, const T&>

constexpr O fill_n( O first, std::iter_difference_t<O> n, const T& value );

(since C++26)

Assigns the given value to all elements in the range [first, first + 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.

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🌸 Parameters 🌸

• first — the beginning of the range of elements to modify
• n — number of elements to modify
• value — the value to be assigned

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🦋 Return value 🦋

An output iterator that compares equal to first + n.

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⭐ Complexity ⭐

Exactly n assignments.

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🧚 Possible implementation 🧚

struct fill_n_fn
{
    template<class O, class T = std::iter_value_t<O>>
    requires std::output_iterator<O, const T&>
    constexpr O operator()(O first, std::iter_difference_t<O> n, const T& value) const
    {
        for (std::iter_difference_t<O> i {}; i != n; ++first, ++i)
            *first = value;
        return first;
    }
};

inline constexpr fill_n_fn fill_n {};

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👑 Notes 👑

• Feature-test macro: __cpp_lib_algorithm_default_value_type — value 202403, std (C++26), feature: List-initialization for algorithms

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💖 Example 💖

// Run this code

#include <algorithm>
#include <complex>
#include <iostream>
#include <string>
#include <vector>

void println(const auto& v)
{
    for (const auto& elem : v)
        std::cout << ' ' << elem;
    std::cout << '\n';
}

int main()
{
    constexpr auto n{8};

    std::vector<std::string> v(n, "▓▓░░");
    println(v);

    std::ranges::fill_n(v.begin(), n, "░░▓▓");
    println(v);

    std::vector<std::complex<double>> nums{{1, 3}, {2, 2}, {4, 8}};
    println(nums);
    #ifdef __cpp_lib_algorithm_default_value_type
        std::ranges::fill_n(nums.begin(), 2, {4, 2});
    #else
        std::ranges::fill_n(nums.begin(), 2, std::complex<double>{4, 2});
    #endif
    println(nums);
}

🌈 Output 🌈

 ▓▓░░ ▓▓░░ ▓▓░░ ▓▓░░ ▓▓░░ ▓▓░░ ▓▓░░ ▓▓░░
 ░░▓▓ ░░▓▓ ░░▓▓ ░░▓▓ ░░▓▓ ░░▓▓ ░░▓▓ ░░▓▓
 (1,3) (2,2) (4,8)
 (4,2) (4,2) (4,8)

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🔥 See also 🔥

• ranges::fill (C++20) — assigns a range of elements a certain value (niebloid)
• ranges::copy_n (C++20) — copies a number of elements to a new location (niebloid)
• ranges::generate (C++20) — saves the result of a function in a range (niebloid)
• ranges::transform (C++20) — applies a function to a range of elements (niebloid)
• ranges::generate_random (C++26) — fills a range with random numbers from a uniform random bit generator (niebloid)
• fill_n — copy-assigns the given value to N elements in a range (function template)

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💖 source http://cppreference.com · version/date 2024.06.10 · page std::ranges::fill_n(3) — mwah, that’s the whole page bestie 🫶✨