std::ranges::fill(3)

NAME
std::ranges::fill - std::ranges::fill

Synopsis
   Defined in header <algorithm>
   Call signature
   template< class T, std::output_iterator<const
   T&> O, std::sentinel_for<O> S > (since C++20)
   constexpr O fill( O first, S last, const T& (until C++26)
   value );
   template< class O, std::sentinel_for<O> S, class
   T = std::iter_value_t<O> >
   requires std::output_iterator<O, const T&> (since C++26)
   constexpr O fill( O first, S last, const T&
   value ); (1)
   template< class T, ranges::output_range<const
   T&> R > (since C++20)
   constexpr ranges::borrowed_iterator_t<R> fill( (until C++26)
   R&& r, const T& value );
   template< class R, class T =
   std::range_value_t<R> > (2)
   requires ranges::output_range<R, const T&> (since C++26)
   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, 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, 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 = std::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 assigns a value to a number of elements
   (C++20) (niebloid)
   ranges::copy
   ranges::copy_if copies a range of elements to a new location
   (C++20) (niebloid)
   (C++20)
   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 copy-assigns the given value to every element in a range
                        (function template)