# std::ranges::fill

From cppreference.com
< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
| (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 ); |  | (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 ); |  | (since C++26)
| (2) |
template< class T, ranges::output_range <const T&> R > constexpr ranges::borrowed_iterator_t <R> fill( R&& r, const T& value ); |  | (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 ); |  | (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 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

Run this code

    #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)

### See also

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)

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