std::uninitialized_default_construct_n(3)

NAME
std::uninitialized_default_construct_n - std::uninitialized_default_construct_n

Synopsis
   Defined in header <memory>
   template< class ForwardIt, class Size >
   ForwardIt uninitialized_default_construct_n( ForwardIt first, Size (1) (since C++17)
   n );
   template< class ExecutionPolicy, class ForwardIt, class Size >
   ForwardIt uninitialized_default_construct_n( ExecutionPolicy&&
   policy, (2) (since C++17)
                        ForwardIt first, Size
   n );
   1) Constructs n objects of type typename std::iterator_traits<ForwardIt>::value_type
   in the uninitialized storage starting at first by default-initialization, as if by
   for (; n > 0; (void) ++first, --n)
       ::new (static_cast<void*>(std::addressof(*first)))
           typename std::iterator_traits<ForwardIt>::value_type;
   If an exception is thrown during the initialization, the objects already constructed
   are destroyed in an unspecified order.
   2) Same as (1), but executed according to policy. This overload participates in
   overload resolution only if
   std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. (until
                        C++20)
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. (since
                        C++20)

Parameters
   first - the beginning of the range of elements to initialize
   n - the number of elements to construct
   policy - the execution policy to use. See execution policy for
                        details.

Type requirements
   -
   ForwardIt must meet the requirements of LegacyForwardIterator.
   -
   No increment, assignment, comparison, or indirection through valid instances of
   ForwardIt may throw exceptions.

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

Complexity
   Linear in n.

Exceptions
   The overload with a template parameter named ExecutionPolicy reports errors as
   follows:
     * If execution of a function invoked as part of the algorithm throws an exception
       and ExecutionPolicy is one of the standard policies, std::terminate is called.
       For any other ExecutionPolicy, the behavior is implementation-defined.
     * If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation
   template<class ForwardIt, class Size>
   ForwardIt uninitialized_default_construct_n(ForwardIt first, Size n)
   {
       using T = typename std::iterator_traits<ForwardIt>::value_type;
       ForwardIt current = first;
       try
       {
           for (; n > 0; (void) ++current, --n)
               ::new (const_cast<void*>(static_cast<const volatile void*>(
                   std::addressof(*current)))) T;
           return current;
       }
       catch (...)
       {
           std::destroy(first, current);
           throw;
       }
   }

Example
// Run this code
 #include <cstring>
 #include <iostream>
 #include <memory>
 #include <string>
 struct S
 {
     std::string m{"default value"};
 };
 int main()
 {
     constexpr int n{3};
     alignas(alignof(S)) unsigned char mem[n * sizeof(S)];
     try
     {
         auto first{reinterpret_cast<S*>(mem)};
         auto last = std::uninitialized_default_construct_n(first, n);
         for (auto it{first}; it != last; ++it)
             std::cout << it->m << '\n';
         std::destroy(first, last);
     }
     catch(...)
     {
         std::cout << "Exception!\n";
     }
     // Notice that for "trivial types" the uninitialized_default_construct_n
     // generally does not zero-initialize the given uninitialized memory area.
     int v[]{1, 2, 3, 4};
     const int original[]{1, 2, 3, 4};
     std::uninitialized_default_construct_n(std::begin(v), std::size(v));
     // An attempt to access v might be an undefined behavior, pending CWG 1997:
     // for (const int i : v)
     // std::cout << i << ' ';
     // The result is unspecified:
     std::cout << (std::memcmp(v, original, sizeof(v)) == 0 ? "un" : "") << "modified\n";
 }

Possible output:
 default value
 default value
 default value
 unmodified
   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
   uninitialized_default_construct default-initialization in an uninitialized
   (C++17) area of memory, defined by a range
                        (function template)
                        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)
                        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)