std::uninitialized_default_construct(3)

NAME
std::uninitialized_default_construct - std::uninitialized_default_construct

Synopsis
   Defined in header <memory>
   template< class ForwardIt >
   void uninitialized_default_construct( ForwardIt first, ForwardIt (1) (since C++17)
   last);
   template< class ExecutionPolicy, class ForwardIt >
   void uninitialized_default_construct( ExecutionPolicy&& policy, (2) (since C++17)
   ForwardIt first, ForwardIt last );
   1) Constructs objects of type typename iterator_traits<ForwardIt>::value_type in the
   uninitialized storage designated by the range [first, last) by
   default-initialization, as if by
 for (; first != last; ++first)
   ::new (/*VOIDIFY*/(*first))
       typename std::iterator_traits<ForwardIt>::value_type;
   where /*VOIDIFY*/(e) is:
      static_cast<void*>(std::addressof(e)) (until C++20)
      const_cast<void*>(static_cast<const volatile (since C++20)
      void*>(std::addressof(e)))
   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 does not participate
   in overload resolution unless
   std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
   (until C++20)
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
   (since C++20) is true.

Parameters
   first, last - the range of the elements to initialize
   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
   (none)

Complexity
   Linear in the distance between first and last.

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>
   void uninitialized_default_construct(ForwardIt first, ForwardIt last)
   {
       using Value = typename std::iterator_traits<ForwardIt>::value_type;
       ForwardIt current = first;
       try {
           for (; current != last; ++current) {
               ::new (const_cast<void*>(static_cast<const volatile void*>(
                   std::addressof(*current)))) Value;
           }
       } catch (...) {
           std::destroy(first, current);
           throw;
       }
   }

Example
// Run this code
 #include <iostream>
 #include <memory>
 #include <string>
 #include <cstring>
 int main()
 {
     struct S { std::string m{ "Default value" }; };
     constexpr int n {3};
     alignas(alignof(S)) unsigned char mem[n * sizeof(S)];
     try
     {
         auto first {reinterpret_cast<S*>(mem)};
         auto last {first + n};
         std::uninitialized_default_construct(first, last);
         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
     // generally does not zero-fill the given uninitialized memory area.
     int v[] { 1, 2, 3, 4 };
     const int original[] { 1, 2, 3, 4 };
     std::uninitialized_default_construct(std::begin(v), std::end(v));
     // for (const int i : v) { std::cout << i << ' '; }
     // Maybe undefined behavior, pending CWG 1997.
     std::cout <<
         (std::memcmp(v, original, sizeof(v)) == 0 ? "Unmodified\n" : "Modified\n");
     // The result is unspecified.
 }

Possible output:
 Default value
 Default value
 Default value
 Unmodified

See also
                        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)
                        constructs objects by value-initialization
   uninitialized_value_construct in an uninitialized area of memory, defined
   (C++17) by a range
                        (function template)
                        constructs objects by default-initialization
   ranges::uninitialized_default_construct in an uninitialized area of memory, defined
   (C++20) by a range
                        (niebloid)