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) Copies value to an uninitialized memory area first + [0, count) 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 all following conditions are satisfied:

`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
-`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.

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.

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
LWG 3870	C++20	this algorithm might create objects on a `const` storage	kept disallowed