Question

I have the following situation

class B {
public:
    B() {};
    virtual ~B() {};
    virtual void seti( int x ) { i = x; };
    virtual void setj( int x ) { j = x; };
    virtual void add() =0;

protected:
    int i;
    int j;
};
class D :  public B {
public:
 virtual void add() { cout << "D-add:" << i + j << endl; }; 
};

class E: public B {
public:
    void seti( int x ) { i = x; };
    void add() { cout << "E-add:" << i + j << endl; }; 
    void mult() { cout << "E-mult:" << i * j << endl; }; 
};

int _tmain(int argc, _TCHAR* argv[])
{
    D *d = new D();
    d->seti(4); d->setj(5); d->add();
    E*e = d;
    e->seti(8); e->add(); e->mult();
    return 0;
}

I get the following error
1>.\CallBack.cpp(38) : error C2440: 'initializing' : cannot convert from 'D *' to 'E *'
1> Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-styl`enter code here`e cast

What i want to do is when I instantiate E, I was use all the information / members of D and do some thing more with it. Should I use hierarchical inheritance like above or should I use multi-level inheritance or is there any other better way. Please advise. Thank you !

Answer 1

I think what you want is a “conversion ctor” – a constructor on E which takes a reference/pointer to D as the parameter and populates itself basing on D’s values. To copy fields defined on B, you can do this:

class B {
public:
....
  B(B& source) { this->i = source.i; this->j = source.j; }
....
}
class E {
public:
....
  E(D& source) : B(source) { // no need to do anything about i,j
  }
....
}

Also, you can write a converter class (DtoEConverter) which will have a function to do that.