std::ranges::uninitialized_default_construct_n(3)

NAME
std::ranges::uninitialized_default_construct_n - std::ranges::uninitialized_default_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_default_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 default-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 skip the objects construction (without changing the observable
   effect) if no non-trivial default constructor is called while default-initializing a
   std::iter_value_t<I> object, which can be detected by
   std::is_trivially_default_constructible_v.

Possible implementation
struct uninitialized_default_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 ValueType = std::remove_reference_t<std::iter_reference_t<I>>;
        if constexpr (std::is_trivially_default_constructible_v<ValueType>)
            return ranges::next(first, n); // skip initialization
        I rollback{first};
        try
        {
            for (; n-- > 0; ++first)
                ::new (const_cast<void*>(static_cast<const volatile void*>
                    (std::addressof(*first)))) ValueType;
            return first;
        }
        catch (...) // rollback: destroy constructed elements
        {
            for (; rollback != first; ++rollback)
                ranges::destroy_at(std::addressof(*rollback));
            throw;
        }
    } }; inline constexpr uninitialized_default_construct_n_fn uninitialized_default_construct_n{};

Example
// Run this code
 #include <cstring>
 #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_default_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_default_construct_n
     // generally does not zero-fill the given uninitialized memory area.
     constexpr int sample[]{1, 2, 3, 4, 5, 6};
     int v[]{1, 2, 3, 4, 5, 6};
     std::ranges::uninitialized_default_construct_n(std::begin(v), std::size(v));
     if (std::memcmp(v, sample, sizeof(v)) == 0)
     {
         // Maybe undefined behavior, pending CWG 1997:
         // for (const int i : v) { std::cout << i << ' '; }
         for (const int i : sample)
             std::cout << i << ' ';
     }
     else
         std::cout << "Unspecified!";
     std::cout << '\n';
 }

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