std::ranges::generate() algorithm

since C++20

Simplified

// (1)
constexpr O generate( O first, S last, F gen );

// (2)
constexpr ranges::borrowed_iterator_t<R> generate( R&& r, F gen );

The type of arguments are generic and have following constraints:

• O - std::input_or_output_iterator
• S - std::sentinel_for<O>
• R - (none)
• F - std::copy_constructible

Additionally, each overload has the following constraints:

• (1) - invocable<F&> && indirectly_writable<O, invoke_result_t<F&>>
• (1) - invocable<F&> && ranges::output_range<R, invoke_result_t<F&>>

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

Detailed

// (1)
template<
  std::input_or_output_iterator O,
  std::sentinel_for<O> S,
  std::copy_constructible F
>
  requires std::invocable<F&> && std::indirectly_writable<O, std::invoke_result_t<F&>>
constexpr O generate( O first, S last, F gen );

// (2)
template<
  class R,
  std::copy_constructible F
>
  requires std::invocable<F&> && ranges::output_range<R, std::invoke_result_t<F&>>
constexpr ranges::borrowed_iterator_t<R> generate( R&& r, F gen );

• (1) Assigns the result of successive invocations of the function object gen to each element in the range [first; last).

• (2) Same as (2), but uses r as the first 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 niebloids.

Parameters

first last	The range of the elements to modify.
r	The range of the elements to modify.
gen	The generator function object.

Return value

An output iterator that compares equal to last.

Complexity

Exactly ranges::distance(first, last) invocations of gen() and assignments.

Exceptions

(none)

Possible implementation

generate(1) and generate(2)

struct generate_fn
{
    template<std::input_or_output_iterator O, std::sentinel_for<O> S,
             std::copy_constructible F>
    requires std::invocable<F&> && std::indirectly_writable<O, std::invoke_result_t<F&>>
    constexpr O operator()(O first, S last, F gen) const
    {
        for (; first != last; *first = std::invoke(gen), ++first)
        {}
        return first;
    }

    template<class R, std::copy_constructible F>
    requires std::invocable<F&> && ranges::output_range<R, std::invoke_result_t<F&>>
    constexpr ranges::borrowed_iterator_t<R> operator()(R&& r, F gen) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::move(gen));
    }
};

inline constexpr generate_fn generate {};

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 device;
    static std::mt19937 engine {device()};
    return distr(engine);
}

void iota(auto& v, int n)
{
    std::ranges::generate(v, [&n]() mutable { return n++; });
}

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

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

    std::ranges::generate(v.begin(), v.end(), dice);
    print("dice: ", v);
    std::ranges::generate(v, dice);
    print("dice: ", v);

    iota(v, 1);
    print("iota: ", v);
}

Output

dice: 4 3 1 6 6 4 5 5
dice: 4 2 5 3 6 2 6 2
iota: 1 2 3 4 5 6 7 8