NAME¶

std::ranges::generate_n - std::ranges::generate_n

Synopsis¶

Defined in header <algorithm>
Call signature
template< std::input_or_output_iterator O, std::copy_constructible F
>

requires std::invocable<F&> && std::indirectly_writable<O, (since C++20)
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, 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 - 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.

Possible implementation¶

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 {};

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 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");
}

Possible output:¶

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

See also¶

ranges::generate saves the result of a function in a range
(C++20) (niebloid)
ranges::generate_random fills a range with random numbers from a uniform random bit
(C++26) generator
(niebloid)
ranges::fill assigns a range of elements a certain value
(C++20) (niebloid)
ranges::fill_n assigns a value to a number of elements
(C++20) (niebloid)
ranges::transform applies a function to a range of elements
(C++20) (niebloid)
assigns the results of successive function calls to N
generate_n elements in a range
(function template)