std::ranges::generate_n() algorithm

od C++20

Simplified

// (1)
constexpr O generate_n( O first, std::iter_difference_t<O> n, F gen );

The type of arguments are generic and have following constraints:

• O - std::input_or_output_iterator
• F - std::copy_constructible

Additionally, each overload has the following constraints:

• (1) - invocable<F&> && indirectly_writable<O, invoke_result_t<F&>>

(The std:: namespace was ommitted here for readability)

Detailed

// (1)
template<
  std::input_or_output_iterator O,
  std::copy_constructible F
>
  requires std::invocable<F&> && std::indirectly_writable<O, std::invoke_result_t<F&>>
constexpr O generate_n( O first, std::iter_difference_t<O> n, F gen );

Assigns the result of successive invocations of the function object gen to each element in the range [first; first + n), if 0 < n.
Does nothing otherwise.

The function-like entities described on this page are niebloids.

Parameters

first	The beginning of the range of elements to modify.
n	Number of elements to modify.
gen	The generator function object.

Return value

Iterator one past the last element assigned if 0 < count, first otherwise.

Complexity

Exactly n invocations of gen() and assignments.

Exceptions

(none)

Possible implementation

generate_n(1)

struct generate_n_fn
{
    template<std::input_or_output_iterator O, std::copy_constructible F>
    requires std::invocable<F&> && std::indirectly_writable<O, std::invoke_result_t<F&>>
    constexpr O operator()(O first, std::iter_difference_t<O> n, F gen) const
    {
        for (; n-- > 0; *first = std::invoke(gen), ++first)
        {}
        return first;
    }
};

inline constexpr generate_n_fn generate_n {};

Examples

Main.cpp

#include <algorithm>
#include <array>
#include <iostream>
#include <random>
#include <string_view>

auto dice()
{
    static std::uniform_int_distribution<int> distr {1, 6};
    static std::random_device engine;
    static std::mt19937 noise {engine()};
    return distr(noise);
}

void print(const auto& v, std::string_view comment)
{
    for (int i : v)
        std::cout << i << ' ';
    std::cout << '(' << comment << ")\n";
}

int main()
{
    std::array<int, 8> v;

    std::ranges::generate_n(v.begin(), v.size(), dice);
    print(v, "dice");

    std::ranges::generate_n(v.begin(), v.size(), [n {0}] mutable { return n++; });
    // same effect as std::iota(v.begin(), v.end(), 0);
    print(v, "iota");
}

Output

5 5 2 2 6 6 3 5 (dice)
0 1 2 3 4 5 6 7 (iota)