C++ Address of this is different when passed to function

user_14100889 0
Viewed 63

I have a very interesting observation that may or may not be expected. I have two very complicated classes that each extend many other unrelated classes. For reference, one is a socket and one is a virtual device. During construction of the virtual device, I want to pass the address of the device to the a socket object. The socket object is also part of the virtual device. It basically looks like this

class TcpSocket : public Socket, public OtherThing {
    void Init(OtherOtherClass *device){
        printf("0x%x\n", device); //0x1aaaf794
    }
};
​
class Device : public OtherClass, public OtherOtherClass {
    Device(){
        printf("0x%x\n", this); //0x1aaaf6d0
        this->socket.Init(this);
        printf("0x%x\n", this); //0x1aaaf6d0
    }
    TcpSocket socket;
};

I cannot reproduce this in a sandbox environment, but when I print out the addresses, they are different. So maybe I will just ask the question.

In function TcpSocket::Init it is expecting an OtherOtherClass* and not a Device*. Is this the reason why the address appears to be different?

c++
2 Answers

Yes.

Device has several base classes. When converting a Device* pointer to a OtherOtherClass* pointer, the value will be different since the new pointer will be pointing at the instance of OtherOtherClass within the instance of Device.

user_103300050

Class Inheritance is an "is-a" relationship. Device contains everything that is in OtherClass, and everything is that in OtherOtherClass. So, a Device object is also an OtherClass object, and is also an OtherOtherClass object.

When you have a Device* pointer (or Device& reference) to a Device object, it points/refers to the Device portion of the object. And when you have an OtherOtherClass* pointer (or OtherOtherClass& reference) to a Device object, it points/refers to the OtherOtherClass portion of the object. And the same with OtherClass.

That is why you see a different memory address being reported in TcpSocket::Init() than you do in Device::Device(). You have two pointers to different portions of the same object.

user_658630
Related