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class Base
{
public:
    int nX;
    int nY;
    Base(){};
    ~Base(){};
};

class Derived
{
private:
    Base pBase;
    int mX;
    int mY;
public:
    Derived(){}
    Derived(int x , int y) { pBase.nX = x; pBase.nY = y;}
    ~Derived(){}
    void Print()
    {
        std::cout<<"Base X is "<< pBase.nX << ", Base Y is " << pBase.nY << std::endl;
    }
};

int main() 
{
    Derived pDerived(10,9);
    Base* pBase = reinterpret_cast<Base*>(&pDerived);
    pDerived.Print();
    std::cout<<"Cast X is "<< pBase->nX<<", Cast Y is "<< pBase->nY<<std::endl;
    return 0;
}

I have come across something above in a Source which did not inherit a Base Class, yet Include it as part of the class/struct, sent to another consumer function as a base pointer, then using reinterpret_cast to cast it back to the Derived class(or another) Depending on the value of nX.

nX is Guaranteed to be unique for every class, At least from what I can see from the source.

I've oversimplified it's usage for the code, and I'm Calling it as Base and Derived due to the lack of a better term since I have absolutely no idea what to call this practice.

The code does seem to grab the proper address, and does output the correct values for nX and nY, and the Applications logic does work as well. The classes doesn't have any virtual functions except for a Default Constructor, unlike my example. I have also kept the structure of the class in my sample, with the member variables above the constructors/member Functions.

My question is, How are objects Stored in memory either automatically or dynamically, Are guaranteed to behave this way(Stored Similar to Structs as stated on an answer on this thread, given the Code above.)? or is this an undefined behavior, and not fixing this would eventually run into segmentation faults or Access Violations?

Azriel Elijay
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    Does this answer your question? [is cast a pointer points-to-class to it's first member illegal?](https://stackoverflow.com/questions/49333703/is-cast-a-pointer-points-to-class-to-its-first-member-illegal) – apple apple Feb 17 '23 at 09:00
  • @appleapple I have read that thread, I posted question since I haven't got the faintest Idea what the Practice is called and The thread's sample was for _dynamic_ objects and I wasn't sure if it would still be the case for automatic objects, Specially on VS2008. – Azriel Elijay Feb 17 '23 at 09:11
  • @‌AzrielElijay pointer-interconvertibility has nothing to do with (automatic/dynamic) storage duration. – apple apple Feb 17 '23 at 14:34
  • I have no idea about what VS2008 would do or if it has any bug, fwiw. – apple apple Feb 17 '23 at 14:35

1 Answers1

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The codes uses composition not inheritance when it keeps an instance of another class as its member. The reinterpret cast only works if the member pBase is the first member.

Once you add another member in front of it, your code will have UB, see example: https://www.godbolt.org/z/3ocr9q5Ts

For more details see is cast a pointer points-to-class to it's first member illegal?

leosch
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