In-place new + reinterpret_cast

Question

The code below compiles, but I am not 100% sure that it conforms to the standard:

#include <memory>

class A
{
public:
    
    int x;
};

int main()
{
    uint8_t m_storage[sizeof(A)];
    
    new (m_storage) A();
    
    auto deleter = [](A * p) { p->~A(); };
    std::unique_ptr<A, decltype(deleter)> p(reinterpret_cast<A *>(m_storage), deleter);
    
    p->x = 0;

    return p->x;
}

Is it a proper usage of reinterpret_cast?

If you can, prefer capturing the pointer produced by the placement new expression. It will take less lawyering. — StoryTeller - Unslander Monica, Dec 11 '20 at 16:05
Related: [What is the purpose of std::launder?](https://stackoverflow.com/questions/39382501/what-is-the-purpose-of-stdlaunder) — IlCapitano, Dec 11 '20 at 16:10

score 1 · Accepted Answer · answered Dec 11 '20 at 16:03

1

Yes it is correct reinterpret cast.

Albeit not correctly aligned storage. And potentially wrong placement new (should cast parameter to void*).

answered Dec 11 '20 at 16:03

Alex Guteniev

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Looks like it can be `alignas(A) uint8_t m_storage[sizeof(A)];` – Dmitriano Dec 11 '20 at 16:19
1

Better would be to use [`std::aligned_storage`](https://en.cppreference.com/w/cpp/types/aligned_storage) instead, eg: `std::aligned_storage_t m_storage;` Or, if you need an array of `A`s: `std::aligned_storage_t m_storage[N];` – Remy Lebeau Dec 11 '20 at 18:29

In-place new + reinterpret_cast

1 Answers1