std::ranges::uninitialized_fill

Header: <memory>

Copies value to an uninitialized memory area [first,last) as if by for (; first != last; ++first) ::new (voidify(*first)) std::remove_reference_t<std::iter_reference_t>(value); return first;

# Declarations

Call signature

template< no-throw-forward-iterator I, no-throw-sentinel-for<I> S,
class T >
requires std::constructible_from<std::iter_value_t<I>, const T&>
I uninitialized_fill( I first, S last, const T& value );

(since C++20) (constexpr since C++26)

template< no-throw-forward-range R, class T >
requires std::constructible_from<ranges::range_value_t<R>,
const T&>
ranges::borrowed_iterator_t<R> uninitialized_fill( R&& r,
const T& value );

(since C++20) (constexpr since C++26)

# Parameters

first, last: iterator-sentinel pair denoting the range of elements to initialize
r: the range of the elements to initialize
value: the value to construct the elements with

# Return value

As described above.

# Notes

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

# Example

#include <iostream>
#include <memory>
#include <string>
 
int main()
{
    constexpr int n{4};
    alignas(alignof(std::string)) char out[n * sizeof(std::string)];
 
    try
    {
        auto first{reinterpret_cast<std::string*>(out)};
        auto last{first + n};
        std::ranges::uninitialized_fill(first, last, "▄▀▄▀▄▀▄▀");
 
        int count{1};
        for (auto it{first}; it != last; ++it)
            std::cout << count++ << ' ' << *it << '\n';
 
        std::ranges::destroy(first, last);
    }
    catch(...)
    {
        std::cout << "Exception!\n";
    }
}

# Defect reports

DR	Applied to	Behavior as published	Correct behavior
LWG 3870	C++20	this algorithm might create objects on a const storage	kept disallowed