std::ranges::generate_n
From cppreference.com
| Defined in header <algorithm>
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| Call signature |
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| template< std::input_or_output_iterator O, std::copy_constructible F > requires std::invocable<F&> && std::indirectly_writable<O, std::invoke_result_t<F&>> |
(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 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
| (C++20) |
saves the result of a function in a range (niebloid) |
| (C++20) |
assigns a range of elements a certain value (niebloid) |
| (C++20) |
assigns a value to a number of elements (niebloid) |
| (C++20) |
applies a function to a range of elements (niebloid) |
| assigns the results of successive function calls to N elements in a range (function template) |