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std::ranges::uninitialized_fill

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& x );
 (1) (since C++20) 
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& x );
 (2) (since C++20)
1) Constructs \(\scriptsize N\)N copies of the given value x in an uninitialized memory area, designated by the range [first, last), where \(\scriptsize N\)N is ranges::distance(first, last).
The function has the effect equivalent to: 
for (; first != last; ++first)
    ::new (static_cast<void*>(std::addressof(*first)))
        std::remove_reference_t<std::iter_reference_t<I>>(x);
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 uses r as the range, as if using ranges::begin(r) as first, and ranges::end(r) as last.

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, 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

An iterator equal to last.

Complexity

\(\scriptsize\mathcal{O}(N)\)π“ž(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, e.g. by using ranges::fill, if the value type of the output range is TrivialType.

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&>
    I operator()(I first, S last, const T& x) const
    {
        I rollback{first};
        try
        {
            for (; !(first == last); ++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;
        }
    }
 
    template<no-throw-forward-range R, class T>
    requires std::constructible_from<ranges::range_value_t<R>, const T&>
    ranges::borrowed_iterator_t<R>
    operator()(R&& r, const T& x) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), x);
    }
};
 
inline constexpr uninitialized_fill_fn uninitialized_fill{};

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, "β–„β–€β–„β–€β–„β–€β–„β–€");
 
        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 β–„β–€β–„β–€β–„β–€β–„β–€
2 β–„β–€β–„β–€β–„β–€β–„β–€
3 β–„β–€β–„β–€β–„β–€β–„β–€
4 β–„β–€β–„β–€β–„β–€β–„β–€

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

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