std::ranges::uninitialized_fill(3)

NAME
std::ranges::uninitialized_fill - std::ranges::uninitialized_fill

Synopsis
   Defined in header <memory>
   Call signature
   template< no-throw-forward-iterator I, no-throw-sentinel-for<I> S, class
   T >
                        (1) (since
   requires std::constructible_from<std::iter_value_t<I>, const T&> C++20)
   I uninitialized_fill( I first, S last, const T& x );
   template< no-throw-forward-range R, class T >
   requires std::constructible_from<ranges::range_value_t<R>, const T&> (2) (since
   ranges::borrowed_iterator_t<R> C++20)
   uninitialized_fill( R&& r, const T& x );
   1) Constructs \(\scriptsize N\)N copies of the given value x in an uninitialized
   memory area, designated by the range [first, last), where \(\scriptsize N\)N is
   ranges::distance(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;
   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, 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 - iterator-sentinel pair denoting the range of elements to initialize
   r - the range of the elements to initialize
   value - the value to construct the elements with

Return value
   An iterator equal to last.

Complexity
   \(\scriptsize\mathcal{O}(N)\)𝓞(N).

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

Notes
   An implementation may improve the efficiency of the ranges::uninitialized_fill, e.g.
   by using ranges::fill, if the value type of the output range is TrivialType.

Possible implementation
   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{};

Example
// Run this code
 #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 ▄▀▄▀▄▀▄▀
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 3870 C++20 this algorithm might create objects on a const kept disallowed
                       storage

See also
   ranges::uninitialized_fill_n copies an object to an uninitialized area of memory,
   (C++20) defined by a start and a count
                        (niebloid)
                        copies an object to an uninitialized area of memory,
   uninitialized_fill defined by a range
                        (function template)