Casting vtable class to void pointer (C++)

Question

I am writing library that is supposed to initialize some data and return void pointer. Pointer is then used in C code. Inside library I want to use C++ class to initialize this data.

void *init()
{
    return new MyData();
}
void fini(void *d)
{
    MyData *m = reinterpret_cast<MyData *>(d);
    delete d;
}

This works while MyData is simple class without virtual methods. But now I want MyData to be an implementation of abstract class.

class Base // No virtual things
{
   int val;
};
class A : public Base // Abstract class
{
    virtual int x() = 0;
};
class B : public A // Implementation
{
    int x() {return val;}
};

If I then cast this class to void*, first 4 bytes of data will not reflect "val" variable of base class. Because of vtable maybe? Fine, I understood that this is wrong way to cast. Any suggestions? Something like

return static_cast<Base *>(new MyData);

seems to work but i also need to reconstruct MyData from void * and delete it properly. C program will only use values from Base part of MyData.

Here is alternative

void *init()
{
    return new MyData();
}
void fini(void *d)
{
    delete reinterpret_cast<MyData *>(d);
}
void *getbase(void *d)
{
    MyData *m = reinterpret_cast<MyData *>(d);
    return static_cast<Base *>(m);
}

but i'd like to avoid third function. Thanks and sorry for my english.

"seems to work but i also need to reconstruct MyData from void * and delete it properly." Is it a problem, or why don't you choose this solution? — geza, Nov 21 '17 at 16:58
Can I delete it this way? delete static_cast(reinterpret_cast(d)); — Sion0, Nov 21 '17 at 17:04
Possible duplicate of [When to use virtual destructors?](https://stackoverflow.com/questions/461203/when-to-use-virtual-destructors) — Simon Kraemer, Nov 21 '17 at 17:59
Thanks. It is more about casting. I was afraid to loose vtable or something when casting to simple Base. — Sion0, Nov 21 '17 at 19:07
@SimonKraemer : No he isn't. The problem is that he has a void *, and he need to delete that - but he needs the full object to do that. — Martin Bonner supports Monica, Nov 23 '17 at 17:42

score 1 · Answer 1 · answered Nov 21 '17 at 17:53

1

delete static_cast<A*>(static_cast<Base *>(d));

Worked in all tests. Thanks to geza.

answered Nov 21 '17 at 17:53

Sion0

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Casting vtable class to void pointer (C++)

1 Answers1