std::ranges::uninitialized_fill() algorithm

since C++20

Simplified

// (1)
I uninitialized_fill( I first, S last, const T& x );

// (2)
ranges::borrowed_iterator_t<R> uninitialized_fill( R&& r, const T& x );

The type of arguments are generic and have following constraints:

• I - no-throw-forward-iterator
• S - no-throw-sentinel-for<I>
• R - no-throw-forward-range
• T - (none)

Additionally, each overload has the following constraints:

• (1) std::constructible_from<std::iter_value_t<I>, const T&>
• (1) std::constructible_from<ranges::range_value_t<R>, const T&>

Detailed

// (1)
template<
  no-throw-forward-iterator I,
  no-throw-sentinel-for<I> S,
  class T
>
  requires std::constructible_from<std::iter_value_t<I>, const T&>
I uninitialized_fill( I first, S last, const T& x );

// (2)
template<
  no-throw-forward-range R,
  class T
>
  requires std::constructible_from<ranges::range_value_t<R>, const T&>
ranges::borrowed_iterator_t<R> uninitialized_fill( R&& r, const T& x );

With the helper types defined as follows:

template< class I, class O >
using uninitialized_fill_result = ranges::in_out_result<I, O>;

• (1) Given N as ranges::distance(first, last):

  Constructs N copies of the given value x in an uninitialized memory area, designated by the range [first; last). The function has the effect equivalent to:

  for (; first != last; ++first)
      ::new (static_cast<void*>(std::addressof(*first)))
          std::remove_reference_t<std::iter_reference_t<I>>(x);
  
  return first;
  caution

  If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.

• (2) Same as (1), but uses r as the 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.

Parameters

first last	The range of elements to fill.
r	The range of elements to fill.
value	The value to construct the elements with.

Return value

An iterator equal to last.

Complexity

Given N as ranges::distance(first, last):

O(N)

Exceptions

The exception thrown on construction of the elements in the destination range, if any.

Possible implementation

uninitialized_fill(1) and uninitialized_fill(2)

struct uninitialized_fill_fn
{
    template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S, class T>
    requires std::constructible_from<std::iter_value_t<I>, const T&>
    I operator()(I first, S last, const T& x) const
    {
        I rollback{first};
        try
        {
            for (; !(first == last); ++first)
                ranges::construct_at(std::addressof(*first), x);
            return first;
        }
        catch (...)
        {
            // rollback: destroy constructed elements
            for (; rollback != first; ++rollback)
                ranges::destroy_at(std::addressof(*rollback));
            throw;
        }
    }

    template<no-throw-forward-range R, class T>
    requires std::constructible_from<ranges::range_value_t<R>, const T&>
    ranges::borrowed_iterator_t<R>
    operator()(R&& r, const T& x) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), x);
    }
};

inline constexpr uninitialized_fill_fn uninitialized_fill{};

Examples

Main.cpp

#include <iostream>
#include <memory>
#include <string>

int main()
{
    constexpr int n{4};
    alignas(alignof(std::string)) char out[n * sizeof(std::string)];

    try
    {
        auto first{reinterpret_cast<std::string*>(out)};
        auto last{first + n};
        std::ranges::uninitialized_fill(first, last, "▄▀▄▀▄▀▄▀");

        int count{1};
        for (auto it{first}; it != last; ++it)
            std::cout << count++ << ' ' << *it << '\n';

        std::ranges::destroy(first, last);
    }
    catch(...)
    {
        std::cout << "Exception!\n";
    }
}

Output

1 ▄▀▄▀▄▀▄▀
2 ▄▀▄▀▄▀▄▀
3 ▄▀▄▀▄▀▄▀
4 ▄▀▄▀▄▀▄▀