std::fill(C++29)

namespace std {
  template <class Dst, class T = Dst::value_type>
  constexpr void fill(const Dst& dst, const T& value); // (1) C++29

  template <class ExecutionPolicy, class Dst, class T = Dst::value_type>
  void fill(ExecutionPolicy&& policy,
            const Dst& dst, const T& value);           // (2) C++29
}

#include <mdspan>
#include <iostream>

int main()
{
  double a[6] = {};

  std::mdspan<double, std::extents<std::size_t, 2, 3>> m{a};
  std::fill(m, 1.5);

  for (double x : a) {
    std::cout << x << ' ';
  }
  std::cout << std::endl;
}