std::uninitialized_fill() algorithm

since C++17

// (1)
template< class ForwardIt, class T >
void uninitialized_fill( ForwardIt first, ForwardIt last, const T& value );

// (2)
template< class ExecutionPolicy, class ForwardIt, class T >
void uninitialized_fill( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, const T& value );

until C++17

// (1)
template< class ForwardIt, class T >
void uninitialized_fill( ForwardIt first, ForwardIt last, 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 */ is

  since C++20

  voidify(e)

  since C++11

  static_cast<void*>(std::addressof(e))

  until C++11

  static_cast<void*>(&e)
  caution

  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.

  Overload Resolution

  These overloads participate in overload resolution only if 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	The beginning of the range of the elements to initialize.
count	The number of elements to construct.
value	The value to construct the elements with.
policy	The execution policy to use. See execution policy for details.

Type requirements

Forwardit	LegacyForwardIterator
ForwardIt NoThrowForwardIt	LegacyForwardIterator

No increment, assignment, comparison, or indirection through valid instances of NoThrowForwardIt may throw exceptions. Applying &* to a NoThrowForwardIt 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 overloads with a template parameter named ExecutionPolicy report 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 none other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

uninitialized_fill (1)

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;
    }
}

Examples

Main.cpp

#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