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std::uninitialized_fill_n

Defined in header `<memory>`
	(1)
template< class ForwardIt, class Size, class T > void uninitialized_fill_n( ForwardIt first, Size count, const T& value );		(until C++11)
template< class ForwardIt, class Size, class T > ForwardIt uninitialized_fill_n( ForwardIt first, Size count, const T& value );		(since C++11)
template< class ExecutionPolicy, class ForwardIt, class Size, class T > ForwardIt uninitialized_fill_n( ExecutionPolicy&& policy, ForwardIt first, Size count, const T& value );		(2)	(since C++17)

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 (static_cast<void*>(std::addressof(*first)))
     typename iterator_traits<ForwardIt>::value_type(x);

If an exception is thrown during the initialization, the function has no effects.

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>> is true

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 `ForwardIterator`.
-No increment, assignment, comparison, or indirection through valid instances of `ForwardIt` may throw exceptions.

Return value

(none)	(until C++11)
Iterator to the element past the last element copied.	(since C++11)

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, std::terminate is called.
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)
{
    typedef typename std::iterator_traits<ForwardIt>::value_type Value;
    ForwardIt current = first;
    try {
        for (; count > 0; ++current, (void) --count) {
            ::new (static_cast<void*>(std::addressof(*current))) Value(value);
        }
        return current;
    } catch (...) {
        for (; first != current; ++first) {
            first->~Value();
        }
        throw;
    }
}

Example

#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

uninitialized_fill	copies an object to an uninitialized area of memory, defined by a range (function template)
std::experimental::parallel::uninitialised_fill_n (parallelism TS)	parallelized version of `std::uninitialised_fill_n` (function template)