std::ranges::uninitialized_value_construct_n(3)

NAME
std::ranges::uninitialized_value_construct_n - std::ranges::uninitialized_value_construct_n

Synopsis
   Defined in header <memory>
   Call signature
   template< no-throw-forward-iterator I >
   requires std::default_initializable<std::iter_value_t<I>> (since C++20)
   I uninitialized_value_construct_n( I first, std::iter_difference_t<I>
   n );
   Constructs n objects of type std::iter_value_t<I> in the uninitialized memory area
   starting at first by value-initialization, as if by
   for (; n-- > 0; ++first)
       ::new (static_cast<void*>(std::addressof(*first)))
           std::remove_reference_t<std::iter_reference_t<I>>();
   If an exception is thrown during the initialization, the objects already constructed
   are destroyed in an unspecified order.
   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 - the beginning of the range of elements to initialize
   n - the number of elements to construct

Return value
   The end of the range of objects (i.e., ranges::next(first, n)).

Complexity
   Linear in 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_value_construct_n, e.g. by using ranges::fill_n, if the value
   type of the range is TrivialType and CopyAssignable.

Possible implementation
   struct uninitialized_value_construct_n_fn
   {
       template<no-throw-forward-iterator I>
       requires std::default_initializable<std::iter_value_t<I>>
       I operator()(I first, std::iter_difference_t<I> n) const
       {
           using T = std::remove_reference_t<std::iter_reference_t<I>>;
           if constexpr (std::is_trivial_v<T> && std::is_copy_assignable_v<T>)
               return ranges::fill_n(first, n, T());
           I rollback{first};
           try
           {
               for (; n-- > 0; ++first)
                   ::new (const_cast<void*>(static_cast<const volatile void*>
                       (std::addressof(*first)))) T();
               return first;
           }
           catch (...) // rollback: destroy constructed elements
           {
               for (; rollback != first; ++rollback)
                   ranges::destroy_at(std::addressof(*rollback));
               throw;
           }
       }
   };
   inline constexpr uninitialized_value_construct_n_fn uninitialized_value_construct_n{};

Example
// Run this code
 #include <iostream>
 #include <memory>
 #include <string>
 int main()
 {
     struct S { std::string m{"█▓▒░ █▓▒░ █▓▒░ "}; };
     constexpr int n{4};
     alignas(alignof(S)) char out[n * sizeof(S)];
     try
     {
         auto first{reinterpret_cast<S*>(out)};
         auto last = std::ranges::uninitialized_value_construct_n(first, n);
         auto count{1};
         for (auto it{first}; it != last; ++it)
             std::cout << count++ << ' ' << it->m << '\n';
         std::ranges::destroy(first, last);
     }
     catch (...)
     {
         std::cout << "Exception!\n";
     }
     // Notice that for "trivial types" the uninitialized_value_construct_n
     // zero-initializes the given uninitialized memory area.
     int v[]{1, 2, 3, 4, 5, 6, 7, 8};
     std::cout << ' ';
     for (const int i : v)
         std::cout << i << ' ';
     std::cout << "\n ";
     std::ranges::uninitialized_value_construct_n(std::begin(v), std::size(v));
     for (const int i : v)
         std::cout << i << ' ';
     std::cout << '\n';
 }

Output:
 1 █▓▒░ █▓▒░ █▓▒░
 2 █▓▒░ █▓▒░ █▓▒░
 3 █▓▒░ █▓▒░ █▓▒░
 4 █▓▒░ █▓▒░ █▓▒░
 1 2 3 4 5 6 7 8
 0 0 0 0 0 0 0 0
   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
                        constructs objects by value-initialization
   ranges::uninitialized_value_construct in an uninitialized area of memory,
   (C++20) defined by a range
                        (niebloid)
                        constructs objects by
   ranges::uninitialized_default_construct default-initialization in an uninitialized
   (C++20) area of memory, defined by a range
                        (niebloid)
                        constructs objects by
   ranges::uninitialized_default_construct_n default-initialization in an uninitialized
   (C++20) area of memory, defined by a start and
                        count
                        (niebloid)
                        constructs objects by value-initialization
   uninitialized_value_construct_n in an uninitialized area of memory,
   (C++17) defined by a start and a count
                        (function template)