Casting object pointer to double void pointer (pointer to pointer to void)

Question

I have a class method in a library (can't change it) which takes a double void pointer as one of its arguments. The declaration is as follows:

bool Queue::pop(void **packet, int &size, unsigned short &sn)

In my application code, I want to pass this function a pointer to another kind of object, in this case a GstBuffer pointer type. It seems like I can do this without any compiler errors if I cast the pointer to (void**) as in the following snippet but I have doubts if that will lead to correct behaviour. Is it valid to convert the pointer like this?

guint16 sn;
int size;
GstBuffer *buf; 
Queue *Q = ... // create a Queue instance
Q->pop((void **)&buf, size, sn);  // is this conversion valid?
size = gst_buffer_get_size(buf);

Answer 1

For all intents and purposes a void pointer can hold addresses of any object (data type), ie it can point to any object, and can be typecasted to any object, your code is valid, I would just use a more idiomatic cast:

Q->pop(reinterpret_cast<void**>(&buf), size, sn);

§7.3.12 Pointer conversions [conv.ptr]

2. A prvalue of type “pointer to cv T ”, where T is an object type, can be converted to a prvalue of type “pointer to cv void ”. The pointer value (6.8.3) is unchanged by this conversion.

Example:

void example(void **packet){
    std::cout << *packet << "\n";                   // pointer value
    std::cout << packet << "\n";                    // pointer address
    std::cout << **reinterpret_cast<int**>(packet); // value
}

int main() 
{
    int* x = new int(20);
    std::cout << x << "\n";                         // pointer value
    std::cout << &x << "\n";                        // pointer address
    example(reinterpret_cast<void**>(&x));       
    delete x;
}

Output:

0xb83eb0
0x7ffc181ab2c8
0xb83eb0
0x7ffc181ab2c8
20

The explicit cast is even only needed because it's a pointer to pointer otherwise the conversion would be implicit, no cast would be needed.