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std::ranges::uninitialized_fill

From cppreference.com
< cpp‎ | memory
 
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Memory management library
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Constrained uninitialized
memory algorithms
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ranges::uninitialized_fill
(C++20)
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Defined in header <memory>
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 );
 (1) (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 );
 (2) (since C++20)
(constexpr since C++26)
1) 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<I>>(value);
return first;

If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.
2) Equivalent to return ranges::uninitialized_fill(ranges::begin(r), ranges::end(r), value);.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

Contents

[edit] 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

[edit] Return value

As described above.

[edit] Complexity

Linear in the size of the uninitialized memory area.

[edit] Exceptions

Any exception thrown on construction of the elements in the destination range.

[edit] 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.

Feature-test macro Value Std Feature
__cpp_lib_raw_memory_algorithms 202411L (C++26) constexpr for specialized memory algorithms, (1,2)

[edit] Possible implementation

struct uninitialized_fill_fn
{
    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&>
    constexpr I operator()(I first, S last, const T& value) const
    {
        I rollback{first};
        try
        {
            for (; !(first == last); ++first)
                ranges::construct_at(std::addressof(*first), value);
            return first;
        }
        catch (...)
        {   
            // rollback: destroy constructed elements
            for (; rollback != first; ++rollback)
                ranges::destroy_at(std::addressof(*rollback));
            throw;
        }
    }
 
    template<no-throw-forward-range R, class T>
        requires std::constructible_from<ranges::range_value_t<R>, const T&>
    constexpr ranges::borrowed_iterator_t<R> operator()(R&& r, const T& value) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), value);
    }
};
 
inline constexpr uninitialized_fill_fn uninitialized_fill{};

[edit] Example

Run this code
#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";
    }
}

Output: 

1 ▄▀▄▀▄▀▄▀
2 ▄▀▄▀▄▀▄▀
3 ▄▀▄▀▄▀▄▀
4 ▄▀▄▀▄▀▄▀

[edit] 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

[edit] See also

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