C ++中的构造方法，继承性，堆栈，堆，此指针和segfaults

Question

I'm struggled over some code where I don't know how to name it and how to solve it. I tried to reduce the code to the following example (so the example itself won't make sense, but it shows the problematic):

 struct MyInterface {
    virtual ~MyInterface() {

    };
    virtual void Output() = 0;
};

class A {
public:
    MyInterface *myInterface;
    A(MyInterface *myInterface) {
        std::cout << "this in A constructor: " << this << std::endl;
        this->myInterface = myInterface;
    }
    void CallA() {
        this->myInterface->Output();
    }
};

class B : public MyInterface, A {
public:
    int v;
    B(int v) : A(this) {
        std::cout << "this in B constructor: " << this << std::endl;
        this->v = v;
    }
    virtual void Output() override {
        std::cout << "Whatever" << std::endl;
    }
    void CallB() {
        std::cout << "this in CallB: " << this << std::endl;
        this->CallA();
    }
};

class Foo {
public:
    B b;
    Foo() : b(42) {
        b = B(41);  //This will make an "invalid" B:
                    //generates B on the Stack but assign the bytes to Foo.b (which is on the the heap)
                    //so b.myInterface will point to the stack
                    //after leaving this context b.other will be invalid
    }
    void Exec() {
        b.CallB();
    }
};
int main(int argc, char **args) {
    Foo *foo = new Foo();
    foo->Exec();    //Gives a segfault, because foo->b.myInterface is not valid
    return 0;
}

First I thought it has something to do with the inheritance and its virtual methods. But I think the main problematic is the this pointer within the constructors.

So my questions: When b is constructed, the this pointer in the constructors points to the stack. Why doesn't show the this pointer to the target memory (in the heap)? No copy constructor is called - Why? How can I Name this problem?

Answer 1

The copy constructor isn't called because you aren't creating a new object you are assigning to an existing object. This calls the assignment operator.

This is copy construction:

B b1(42); // construction
B b2(b1); // copy construction
B b3 = b1; // looks like assignment but is actually copy construction

This is assignment:

B b1(42); // construction
b1 = B(43); // b1 already exists we can't copy construct, construct a new object and assign to b1

You need to override the assignment operator :

class B
{
   B& operator=(const B& other)
   {
      // fix references to this here
   }
}