std::ranges::generate_n

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, std::invoke_result_t<F&>> constexpr O generate_n( O first, std::iter_difference_t<O> n, F gen );		(since C++20)

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 may not be specified when calling any of them.
None of them is visible to argument-dependent lookup.
When one of them is found by normal unqualified lookup for the name to the left of the function-call operator, it inhibits argument-dependent lookup.

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)

Compiler support
Freestanding and hosted
Language
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries

ranges::generate (C++20)	saves the result of a function in a range (niebloid)
ranges::fill (C++20)	assigns a range of elements a certain value (niebloid)
ranges::fill_n (C++20)	assigns a value to a number of elements (niebloid)
ranges::transform (C++20)	applies a function to a range of elements (niebloid)
generate_n	assigns the results of successive function calls to N elements in a range (function template)