Zero sized array in struct managed by shared pointer

Question

Consider the following structure:

struct S
{
    int a;
    int b;
    double arr[0];
} __attribute__((packed));

As you can see, this structure is packed and has Zero sized array at the end. I'd like to send this as binary data over the network (assume I took care of endianity).

In C/C++ I could just use malloc to allocate as much space as I want and use free later.

I'd like this memory to be handled by std::shared_ptr.

Is there a straight forward way of doing so without special hacks?

Answer 1

I'd like this memory to be handled by std::shared_ptr.

Is there a straight forward way of doing so without special hacks?

Sure, there is:

shared_ptr<S> make_buffer(size_t s)
{
    auto buffer = malloc(s);                    // allocate as usual
    auto release = [](void* p) { free(p); };    // a deleter
    shared_ptr<void> sptr(buffer, release);     // make it shared
    return { sptr, new(buffer) S };             // an aliased pointer
}

This works with any objects that are placed in a malloced buffer, not just when there are zero-sized arrays, provided that the destructor is trivial (performs no action) because it is never called.

The usual caveats about zero-sized arrays and packed structures still apply as well, of course.