std::ranges::fill() algorithm

since C++20

Simplified

// (1)
template<
  class T,
  std::output_iterator<const T&> O,
  std::sentinel_for<O> S
>
constexpr O fill( O first, S last, const T& value );

// (2)
template<
  class T,
  ranges::output_range<const T&> R
>
constexpr ranges::borrowed_iterator_t<R> fill( R&& r, const T& value );

The type of arguments are generic and have the following constraints:

• T - (none)
• O - std::output_iterator<const T&>
• S - std::sentinel_for<O>
• (2) - R - std::ranges::output_range<const T&>

Detailed

// (1)
template<
  class T,
  std::output_iterator<const T&> O,
  std::sentinel_for<O> S
>
constexpr O fill( O first, S last, const T& value );

// (2)
template<
  class T,
  ranges::output_range<const T&> R
>
constexpr ranges::borrowed_iterator_t<R> fill( R&& r, const T& value );

• (1) Assigns the given value to the elements in the range [first; last).
• (2) Same as (1), but uses r as the source 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.

Parameters

first last	The range of elements to examine.
r	The range of elements to examine.
value	The value to assign.

Return value

An output iterator that compares equal to last.

Complexity

Exactly last - first assignments.

Exceptions

(none)

Possible implementation

ranges::fill

struct fill_fn
{
    template<class T, std::output_iterator<const T&> O, std::sentinel_for<O> S>
    constexpr O operator()(O first, S last, const T& value) const
    {
        while (first != last)
            *first++ = value;

        return first;
    }

    template<class T, ranges::output_range<const T&> R>
    constexpr ranges::borrowed_iterator_t<R> operator()(R&& r, const T& value) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), value);
    }
};

inline constexpr fill_fn fill;

Examples

The following code uses ranges::fill to set all elements of std::vector<int> first to -1, then to 10.

Main.cpp

#include <algorithm>
#include <iostream>
#include <vector>

void println(std::vector<int> const& vi)
{
    for (int e : vi)
        std::cout << e << ' ';
    std::cout << '\n';
}

int main()
{
    std::vector<int> v {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};

    std::ranges::fill(v.begin(), v.end(), -1);
    println(v);

    std::ranges::fill(v, 10);
    println(v);
}

Output

-1 -1 -1 -1 -1 -1 -1 -1 -1 -1
10 10 10 10 10 10 10 10 10 10