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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 (/*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) (until C++20)
`const_cast<void>(static_cast<const volatile void>(std::addressof(e)))`	(since C++20)

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	-	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

(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 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 (const_cast<void*>(static_cast<const volatile void*>(
                std::addressof(*current)))) V(value);
        }
        return current;
    } catch (...) {
        for (; first != current; ++first) {
            first->~V();
        }
        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)
ranges::uninitialized_fill_n (C++20)	copies an object to an uninitialized area of memory, defined by a start and a count (niebloid)