Casting Class Pointer to Void Pointer

Question

How can I able to cast a class pointer to a generic pointer like void*? Like is this code valid?,

class CFoo
{
   int a;
public:
   CFoo():a(1){}
   ~CFoo(){}
   getNum(){return a;}
};

void tfunc(void* data)
{
    CFoo* foo = static_cast<CFoo*>(data);
    std::cout << "Number: " << foo->getNum();
    delete foo;
}

int main()
{
   CFoo* foo = new CFoo;
   void* dt = static_cast<void*>(foo);
   tfunc(dt); // or tfunc(static_cast<void*>(food));

   return 0;
}

Answer 1

This is perfectly valid. Here is what standard has to say about it:

§4.10 Pointer conversions

2 An rvalue of type "pointer to cv T ," where T is an object type, can be converted to an rvalue of type "pointer to cv void ." The result of converting a "pointer to cv T " to a "pointer to cv void " points to the start of the storage location where the object of type T resides, as if the object is a most derived object (1.8) of type T (that is, not a base class subobject).

which means you can convert your pointer to class to a void pointer. And ...

§5.2.9 Static cast

10 An rvalue of type "pointer to cv void " can be explicitly converted to a pointer to object type. A value of type pointer to object converted to "pointer to cv void " and back to the original pointer type will have its original value.

which means you can use static_cast to convert a void pointer back to an original class pointer.

Hope it helps. Good Luck!

Answer 2

In C++ you don't need the static cast to get to void* :

int main()
{
    CFoo* foo = new CFoo;
    void* dt = foo;
    tfunc(dt); // or tfunc(foo);

    return 0;
}

NB: your implementation of tfunc() is quite correct in that it does need the cast.

Answer 3

Is this valid?

Yes, it is valid as per standard § 5.2.9.7

A prvalue of type “pointer to cv1 void” can be converted to a prvalue of type “pointer to cv2 T,” where T is an object type and cv2 is the same cv-qualification as, or greater cv-qualification than, cv1. The null pointer value is converted to the null pointer value of the destination type. A value of type pointer to object converted to “pointer to cv void” and back, possibly with different cv-qualification, shall have its original value. [ Example:

T* p1 = new T;
const T* p2 = static_cast<const T*>(static_cast<void*>(p1));
bool b = p1 == p2; // b will have the value true.

Answer 4

    CFoo* foo = new CFoo;
    void* dt = (void*)foo;