奇怪的访问冲突错误

Question

I have wrote a program that use inheritance, everything is OK but an error that I think shouldn't not be here naturally. here is my program:

class A
{
protected:
    int x;
public:
    A(){};
    ~A(){};
    A* operator=(const A* other)
    {
        this->x = other->x;
        return this;
    }
};
class B : public A
{
public:
    B() : A(){};
    ~B(){};
};
class C
{
protected:
    A *a;
public:
    C(const A* other){ *(this->a) = other; };
};

int main()
{
    B *b = new B();
    C *c = new C(b);
    return 0;
}

It produces an execution time error in the statement 'this->x = other->x;'. What about this?

Answer 1

*(this->a) is undefined behavior, because this->a wasn't initialized - it's just a dangling pointer.

You can use a = new A; *a = other a = new A; *a = other (the this-> in this case is redundant), but this isn't the proper C++ way - you should use RAII (look it up) - you wouldn't need destructors, assignment operators or copy constructors if you did.

Also, operator = typically returns *this by reference.

Answer 2

I like Luchian's answer but there is still some places to enhanced in your code, you may still encounter some undefined behavior in your code before you fix other potential bugs.

class A
{
protected:
    int x;
public:
    A():x(0){}     // should always initialize x in constructor
    virtual ~A()=0 {} // as A is a base case, ~A must be virtual
    A& operator=(const A& other)  // a reference will be more practical better?
    {
        this->x = other.x;
        return *this;
    }
};

If you don't want to make B object as well which means B is serviced as a base class, you should make its destructor virtual as well, even make it pure virtual.

class B : public A
{
public:
    B() : A(){}
    virtual ~B(){} =0
    { }       
};

*(this->a) = other; is answered by Luchian already. In your code, if you want to keep a copy of pointer which points to A, you can simply initialize a_ptr in member initialize list see below demo code:

class C
{
protected:
   A *a_ptr;
public:
    C(A* other):a_ptr(other){ }
};

Finally come to your main function, if b,c are only used in main function, when program finishes, system will claim dynamic allocated memory back, but if you use a,b in a loop, you need to delete them manually.