std::ranges::uninitialized_value_construct() algorithm

od C++20

Simplified

// (1)I uninitialized_value_construct( I first, S last );// (2)ranges::borrowed_iterator_t<R>  uninitialized_value_construct( R&& r );

The type of arguments are generic and have following constraints:

• I - no-throw-forward-iterator
• S - no-throw-sentinel-for<I>
• R - no-throw-forward-range

Additionally, each overload has the following constraints:

• (1) std::default_initializable<std::iter_value_t<I>>
• (2) std::default_initializable<ranges::range_value_t<R>>

Detailed

// (1)template<  no-throw-forward-iterator I,  no-throw-sentinel-for<I> S>  requires std::default_initializable<std::iter_value_t<I>>I uninitialized_value_construct( I first, S last );// (2)template< no-throw-forward-range R >  requires std::default_initializable<ranges::range_value_t<R>>ranges::borrowed_iterator_t<R>  uninitialized_value_construct( R&& r );

Constructs n objects of type std::iter_value_t<I> in the uninitialized memory area starting at first by value-initialization, as if by:

for (; n-- > 0; ++first)    ::new (static_cast<void*>(std::addressof(*first)))        std::remove_reference_t<std::iter_reference_t<I>>();

uwaga

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.

Parameters

first	The beginning of the range of elements to initialize.
n	The number of elements to construct.

Return value

The end of the range of objects (i.e., ranges::next(first, n)).

Complexity

Linear in the distance between first and last.

Exceptions

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

Possible implementation

uninitialized_value_construct(1)

struct uninitialized_value_construct_n_fn{    template<no-throw-forward-iterator I>    requires std::default_initializable<std::iter_value_t<I>>    I operator()(I first, std::iter_difference_t<I> n) const    {        using T = std::remove_reference_t<std::iter_reference_t<I>>;        if constexpr (std::is_trivial_v<T> && std::is_copy_assignable_v<T>)            return ranges::fill_n(first, n, T());        I rollback{first};        try        {            for (; n-- > 0; ++first)                ::new (const_cast<void*>(static_cast<const volatile void*>                    (std::addressof(*first)))) T();            return first;        }        catch (...) // rollback: destroy constructed elements        {            for (; rollback != first; ++rollback)                ranges::destroy_at(std::addressof(*rollback));            throw;        }    }};inline constexpr uninitialized_value_construct_n_fn uninitialized_value_construct_n{};

Notes

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

Examples

Main.cpp

#include <iostream>#include <memory>#include <string>int main(){    struct S { std::string m{ "█▓▒░ █▓▒░ █▓▒░ " }; };    constexpr int n{4};    alignas(alignof(S)) char out[n * sizeof(S)];    try    {        auto first{reinterpret_cast<S*>(out)};        auto last = std::ranges::uninitialized_value_construct_n(first, n);        auto count{1};        for (auto it{first}; it != last; ++it)            std::cout << count++ << ' ' << it->m << '\n';        std::ranges::destroy(first, last);    }    catch (...)    {        std::cout << "Exception!\n";    }    // Notice that for "trivial types" the uninitialized_value_construct_n    // zero-initializes the given uninitialized memory area.    int v[]{1, 2, 3, 4, 5, 6, 7, 8};    std::cout << ' ';    for (const int i : v)        std::cout << i << ' ';    std::cout << "\n ";    std::ranges::uninitialized_value_construct_n(std::begin(v), std::size(v));    for (const int i : v)        std::cout << i << ' ';    std::cout << '\n';}

Output

1 █▓▒░ █▓▒░ █▓▒░ 2 █▓▒░ █▓▒░ █▓▒░ 3 █▓▒░ █▓▒░ █▓▒░ 4 █▓▒░ █▓▒░ █▓▒░1 2 3 4 5 6 7 80 0 0 0 0 0 0 0