Defined in header <algorithm>
Call signature
template< class T, std::output_iterator<const T&> O, std::sentinel_for<O> S >
constexpr O
    fill( O first, S last, const T& value );
(1) (since C++20)
template< class T, ranges::output_range<const T&> R >
constexpr ranges::borrowed_iterator_t<R>
    fill( R&& r, const T& value );
(2) (since C++20)
1) Assigns the given value to the elements in the range [firstlast).
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, that is:

In practice, they may be implemented as function objects, or with special compiler extensions.


first, last - the range of elements to modify
r - the range of elements to modify
value - the value to be assigned

Return value

An output iterator that compares equal to last.


Exactly last - first assignments.

Possible implementation

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;


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

#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);
    std::ranges::fill(v, 10);


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

See also

assigns a value to a number of elements
copies a range of elements to a new location
saves the result of a function in a range
applies a function to a range of elements
copy-assigns the given value to every element in a range
(function template)

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