std::ranges::destroy_n() algorithm

od C++20

Simplified

// (1)
constexpr I destroy_n( I first, std::iter_difference_t<I> n ) noexcept;

The type of arguments are generic and have following constraints:

• I - no-throw-input-iterator

Additionally, each overload has the following constraints:

• (1) std::destructible<std::iter_value_t<I>>

Detailed

// (1)
template< no-throw-input-iterator I >
    requires std::destructible<std::iter_value_t<I>>
constexpr I destroy_n( I first, std::iter_difference_t<I> n ) noexcept;

Destroys the n objects in the range starting at first, equivalent to

return std::ranges::destroy(std::counted_iterator(first, n), std::default_sentinel).base();

The function-like entities described on this page are niebloids.

Parameters

first	The beginning of the range of elements to destroy.
n	The number of elements to destroy.

Return value

The end of the range of objects that has been destroyed.

Complexity

Linear in n.

Exceptions

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

Possible implementation

destroy_n(1)

struct destroy_n_fn
{
    template<no-throw-input-iterator I>
        requires std::destructible<std::iter_value_t<I>>
    constexpr I operator()(I first, std::iter_difference_t<I> n) const noexcept
    {
        for (; n != 0; (void)++first, --n)
            std::ranges::destroy_at(std::addressof(*first));
        return first;
    }
};

inline constexpr destroy_n_fn destroy_n{};

Examples

Main.cpp

#include <iostream>
#include <memory>
#include <new>

struct Tracer
{
    int value;
    ~Tracer() { std::cout << value << " destructed\n"; }
};

int main()
{
    alignas(Tracer) unsigned char buffer[sizeof(Tracer) * 8];

    for (int i = 0; i < 8; ++i)
        new(buffer + sizeof(Tracer) * i) Tracer{i}; //manually construct objects

    auto ptr = std::launder(reinterpret_cast<Tracer*>(buffer));

    std::ranges::destroy_n(ptr, 8);
}

Output

0 destructed
1 destructed
2 destructed
3 destructed
4 destructed
5 destructed
6 destructed
7 destructed