std::ranges::fill

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) (until C++26)
template< class O, std::sentinel_for<O> S, class T = std::iter_value_t<O> > requires std::output_iterator<O, const T&> constexpr O fill( O first, S last, const T& value );	(1)		(since C++26)
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) (until C++26)
template< class R, class T = ranges::range_value_t<R> > requires ranges::output_range<R, const T&> constexpr ranges::borrowed_iterator_t<R> fill( R&& r, const T& value );	(2)		(since C++26)

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 algorithm function objects (informally known as 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.

Parameters

first, last	-	the iterator-sentinel pair defining 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.

Complexity

Exactly last - first assignments.

Possible implementation

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

    template<class R, class T = ranges::range_value_t<R>>
    requires ranges::output_range<R, const T&>
    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;

Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_algorithm_default_value_type`	`202403`	(C++26)	List-initialization for algorithms (1,2)

Example

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

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

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

    // set all elements to -1 using overload (1)
    std::ranges::fill(v.begin(), v.end(), -1);
    println(v);

    // set all element to 10 using overload (2)
    std::ranges::fill(v, 10);
    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(nums, {4, 2}); // T gets deduced
    #else
        std::ranges::fill(nums, std::complex<double>{4, 2});
    #endif
    println(nums);
}

Output:

-1 -1 -1 -1 -1 -1
10 10 10 10 10 10
(1,3) (2,2) (4,8)
(4,2) (4,2) (4,2)

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