NAME

std::ranges::generate - std::ranges::generate

Synopsis

Defined in header <algorithm>
Call signature
template< std::input_or_output_iterator O, std::sentinel_for<O> S,

std::copy_constructible F >
requires std::invocable<F&> && std::indirectly_writable<O, (1) (since C++20)
std::invoke_result_t<F&>>
constexpr O

generate( O first, S last, F gen );
template< class R, std::copy_constructible F >

requires std::invocable<F&> && ranges::output_range<R,
std::invoke_result_t<F&>> (2) (since C++20)
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 (1), but uses r as the 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, 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.

Parameters

first, last - the range of elements to modify
r - the range of 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.

Possible implementation

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 {};

Example

// Run this code

#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);
}

Possible 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