NAME

std::ranges::fill_n - std::ranges::fill_n

Synopsis

Defined in header <algorithm>
Call signature
template< class T, std::output_iterator<const T&> O > (since C++20)
constexpr O fill_n( O first, std::iter_difference_t<O> n, const T& (until C++26)
value );
template< class O, class T = std::iter_value_t<O> >

requires std::output_iterator<O, const T&> (since C++26)

constexpr O fill_n( O first, std::iter_difference_t<O> n, const T&
value );

Assigns the given value to all elements in the range [first, first + n).

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
value - the value to be assigned

Return value

An output iterator that compares equal to first + n.

Complexity

Exactly n assignments.

Possible implementation

struct fill_n_fn
{
template<class O, class T = std::iter_value_t<O>>
requires std::output_iterator<O, const T&>
constexpr O operator()(O first, std::iter_difference_t<O> n, const T& value) const
{
for (std::iter_difference_t<O> i {}; i != n; ++first, ++i)
*first = value;
return first;
}
};

inline constexpr fill_n_fn fill_n {};

Notes

Feature-test macro Value Std Feature
__cpp_lib_algorithm_default_value_type 202403 (C++26) List-initialization for
algorithms

Example

// Run this code

#include <algorithm>
#include <complex>
#include <iostream>
#include <string>
#include <vector>

void println(const auto& v)
{
for (const auto& elem : v)
std::cout << ' ' << elem;
std::cout << '\n';
}

int main()
{
constexpr auto n{8};

std::vector<std::string> v(n, "▓▓░░");
println(v);

std::ranges::fill_n(v.begin(), n, "░░▓▓");
println(v);

std::vector<std::complex<double>> nums{{1, 3}, {2, 2}, {4, 8}};
println(nums);
#ifdef __cpp_lib_algorithm_default_value_type
std::ranges::fill_n(nums.begin(), 2, {4, 2});
#else
std::ranges::fill_n(nums.begin(), 2, std::complex<double>{4, 2});
#endif
println(nums);
}

Output:

▓▓░░ ▓▓░░ ▓▓░░ ▓▓░░ ▓▓░░ ▓▓░░ ▓▓░░ ▓▓░░
░░▓▓ ░░▓▓ ░░▓▓ ░░▓▓ ░░▓▓ ░░▓▓ ░░▓▓ ░░▓▓
(1,3) (2,2) (4,8)
(4,2) (4,2) (4,8)

See also

ranges::fill assigns a range of elements a certain value
(C++20) (niebloid)
ranges::copy_n copies a number of elements to a new location
(C++20) (niebloid)
ranges::generate saves the result of a function in a range
(C++20) (niebloid)
ranges::transform applies a function to a range of elements
(C++20) (niebloid)
ranges::generate_random fills a range with random numbers from a uniform random bit
(C++26) generator
(niebloid)
fill_n copy-assigns the given value to N elements in a range
(function template)