std::uninitialized_fill_n - cppreference.com

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Defined in header `<memory>`
template< class NoThrowForwardIt, class Size, class T > NoThrowForwardIt uninitialized_fill_n( NoThrowForwardIt first, Size count, const T& value );	(1)	(constexpr since C++26)
template< class ExecutionPolicy, class NoThrowForwardIt, class Size, class T > NoThrowForwardIt uninitialized_fill_n( ExecutionPolicy&& policy, NoThrowForwardIt first, Size count, const T& value );	(2)	(since C++17)

1) Constructs the first count elements in the destination range beginning at first with the given value value as if by

for (; count--; ++first)
::new (voidify(*first))
typename std::iterator_traits<NoThrowForwardIt>::value_type(value);
return first;

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 the value of the following expression is true:

`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)

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
-`NoThrowForwardIt` must meet the requirements of 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

As described above.

Exceptions

2) During the execution process:

If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).

Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_raw_memory_algorithms`	`202411L`	(C++26)	`constexpr` for specialized `<memory>` algorithms, (1)

Possible implementation

template<class NoThrowForwardIt, class Size, class T>
constexpr NoThrowForwardIt uninitialized_fill_n(NoThrowForwardIt first,
                                                Size count, const T& value)
{
    using V = typename std::iterator_traits<NoThrowForwardIt>::value_type;
    NoThrowForwardIt 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;
    }
    return current;
}

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

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 866	C++98	given `T` as the value type of `NoThrowForwardIt`, if `T::operator new` exists, the program might be ill-formed	uses global placement `new` instead
LWG 1339	C++98	the location of the first element following the filling range was not returned	returned
LWG 2433	C++11	this algorithm might be hijacked by overloaded `operator&`	uses std::addressof