std::uninitialized_fill_n(3)

NAME
std::uninitialized_fill_n - std::uninitialized_fill_n

Synopsis
   Defined in header <memory>
   template< class ForwardIt, class Size, class T >
   ForwardIt uninitialized_fill_n( ForwardIt first, Size count, const (1)
   T& value );
   template< class ExecutionPolicy, class ForwardIt, class Size,
   class T >
   ForwardIt uninitialized_fill_n( ExecutionPolicy&& policy, (2) (since C++17)
                        ForwardIt first, Size count, const
   T& value );
   1) Copies the given value value to the first count elements in an uninitialized
   memory area beginning at first as if by for (; n--; ++first)
       ::new (/* VOIDIFY */(*first))
           typename std::iterator_traits<ForwardIt>::value_type(value);
   where /* VOIDIFY */(e) is:
   static_cast<void*>(&e) (until C++11)
   static_cast<void*>(std::addressof(e)) (since C++11)
   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 the elements to
                        initialize
   count - number of elements to construct
   value - the value to construct the elements with

Type requirements
   -
   ForwardIt must meet the requirements of LegacyForwardIterator.
   -
   No increment, assignment, comparison, or indirection through valid instances of
   ForwardIt may throw exceptions.
   Applying &* to a ForwardIt value must yield a pointer to its value type.
   (until C++11)

Return value
   Iterator to the element past the last element copied.

Complexity
   Linear in count.

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, class T>
   ForwardIt uninitialized_fill_n(ForwardIt first, Size count, const T& value)
   {
       using V = typename std::iterator_traits<ForwardIt>::value_type;
       ForwardIt current = first;
       try
       {
           for (; count > 0; ++current, (void) --count)
               ::new (static_cast<void*>(std::addressof(*current))) V(value);
           return current;
       }
       catch (...)
       {
           for (; first != current; ++first)
               first->~V();
           throw;
       }
   }

Example
// Run this code
 #include <algorithm>
 #include <iostream>
 #include <memory>
 #include <string>
 #include <tuple>
 int main()
 {
     std::string* p;
     std::size_t sz;
     std::tie(p, sz) = std::get_temporary_buffer<std::string>(4);
     std::uninitialized_fill_n(p, sz, "Example");
     for (std::string* i = p; i != p + sz; ++i)
     {
         std::cout << *i << '\n';
         i->~basic_string<char>();
     }
     std::return_temporary_buffer(p);
 }

Output:

Example

Example

Example

Example
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
                       given T as the value type of ForwardIt,
   LWG 866 C++98 if uses global replacement-
                       T::operator new exists, the program new instead
                       might be ill-formed
                       the location of the first element
   LWG 1339 C++98 following returned
                       the filling range was not returned
   LWG 2433 C++11 this algorithm might be hijacked by uses std::addressof
                       overloaded operator&
   LWG 3870 C++20 this algorithm might create objects on kept disallowed
                       a const storage

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