std::destroy_at() algorithm

od C++17

// (1)
template< class T >
void destroy_at( T* p );

If T is not an array type, calls the destructor of the object pointed to by p, as if by p->~T(). If T is an array type, the program is ill-formed (do C++20) recursively destroys elements of *p in order, as if by calling std::destroy(std::begin(*p), std::end(*p)) (od C++20).

Parameters

p	A pointer to the object to be destroyed.

Return value

(none)

Complexity

O(1)

Exceptions

(none)

Possible implementation

destroy_at(1)

template<class T>
constexpr void destroy_at(T* p)
{
    if constexpr (std::is_array_v<T>)
        for (auto &elem : *p)
            (destroy_at)(std::addressof(elem));
    else
        p->~T();
}
// C++17 version:
// template<class T> void destroy_at(T* p) { p->~T(); }

Notes

destroy_at deduces the type of object to be destroyed and hence avoids writing it explicitly in the destructor call.

od C++20

When destroy_at is called in the evaluation of some constant expression e, the argument p must point to an object whose lifetime began within the evaluation of e.

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));

    for (int i = 0; i < 8; ++i)
        std::destroy_at(ptr + i);
}

Output

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