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std::ranges::uninitialized_fill

From cppreference.com
< cpp‎ | memory
 
 
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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 ranges::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>

ranges::uninitialized_fill( R&& r, const T& x );
(2) (since C++20)
1) Constructs N copies of the given value x in an uninitialized memory area, defined by the range [first, last), where N = ranges::distance(first, last).
The function has the effect equivalent to:
for (; first != last; ++first) {
    ::new (
        const_cast<void*>(static_cast<const volatile 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.

Contents

[edit] Parameters

first, last - the range of the elements to initialize
r - the range of the elements to initialize
value - the value to construct the elements with.

[edit] Return value

An iterator equal to last.

[edit] Complexity

π“ž(N).

[edit] Notes

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

[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&>
    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{};

[edit] 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–€

[edit] See also

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