std::ranges::uninitialized_fill_n

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Defined in header <memory>
Call signature
template< no-throw-forward-range I, class T >

requires std::constructible_from<std::iter_value_t<I>, const T&>

I uninitialized_fill_n( I first, std::iter_difference_t<I> n, const T& x );
 (since C++20)

Constructs n copies of the given value x in an uninitialized memory area, designated by the range [first, first + n), as if by 

for (; n--; ++first) {
    ::new (static_cast<void*>(std::addressof(*first)))
        std::remove_reference_t<std::iter_reference_t<I>>(x);
}

If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.

The function-like entities described on this page are niebloids, that is:

In practice, they may be implemented as function objects, or with special compiler extensions.

Parameters

first - the beginning of the range of the elements to initialize
n - number of elements to construct
x - the value to construct the elements with

Return value

An iterator equal to first + n.

Complexity

Linear in n.

Exceptions

The exception thrown on construction of the elements in the destination range, if any.

Notes

An implementation may improve the efficiency of the ranges::uninitialized_fill_n, e.g. by using ranges::fill_n, if the value type of the output range is TrivialType.

Possible implementation

struct uninitialized_fill_n_fn
{
    template<no-throw-forward-range I, class T>
    requires std::constructible_from<std::iter_value_t<I>, const T&>
    I operator()(I first, std::iter_difference_t<I> n, const T& x) const
    {
        I rollback{first};
        try
        {
            for (; n-- > 0; ++first)
                ranges::construct_at(std::addressof(*first), x);
            return first;
        }
        catch (...) // rollback: destroy constructed elements
        {
            for (; rollback != first; ++rollback)
                ranges::destroy_at(std::addressof(*rollback));
            throw;
        }
    }
};
 
inline constexpr uninitialized_fill_n_fn uninitialized_fill_n{};

Example

Run this code
#include <iostream>
#include <memory>
#include <string>
 
int main()
{
    constexpr int n{3};
    alignas(alignof(std::string)) char out[n * sizeof(std::string)];
 
    try
    {
        auto first{reinterpret_cast<std::string*>(out)};
        auto last = std::ranges::uninitialized_fill_n(first, n, "cppreference");
 
        for (auto it{first}; it != last; ++it)
            std::cout << *it << '\n';
 
        std::ranges::destroy(first, last);
    }
    catch (...)
    {
        std::cout << "Exception!\n";
    }
}

Output: 

cppreference
cppreference
cppreference

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

See also

(C++20)
 copies an object to an uninitialized area of memory, defined by a range
(niebloid)
copies an object to an uninitialized area of memory, defined by a start and a count
(function template)
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