Construct an object from its base

[DBarzo]

Hi,

I have this situation:

class Base {
private:
  map<UINT, string> m_MyMap;
  string m_MyString;

public:
  Base() {};
  
  Base(string str) { 
    ParseString(str); 
  }

  void ParseString(string& str) {
     //...populare map
  }
};


class Derived : public Base {
public:
  Derived(string str) : Base(str) {};

};

Now, I'm searching for a way to create a Derived object from the Base (best without calling ParseString() but using the map directly), something like:

Base BaseObj("bla bla");
Derived dev(BaseObj);

or

Base BaseObj("bla bla");
Derived dev = BaseObj;

How can I do it?

Regards,
Daniele.

[Moschops]

Sounds like you're trying to create a constructor for the derived class

Derived::Derived(Base  someBaseObject)
{
// code in here to copy the fields you want to copy
  }

[clanmjc]

What Moschops said, you will also want to write a copy constructor for the Base class. _{If you want to do it right, using the member initialize list}

[therockon7throw]

You dont have to do any thing at all, your sub-class constructor does the work for you already, whenever you create a new object of sub-class, that is the way it works:-)

Derived(string str) : Base(str) {}; dont you see ? your sub-class call the constructor of your super-class...

you create an object like this Derived dev ("bla bla"); that is all

[DBarzo]

Hi guys,

thanks for your replies!
Yes, at the end, I need to write a copy constructor but, writing it:

class Base {
private:
  map<UINT, string> m_MyMap;
  string m_MyString;

public:
  Base() {};
  
  Base(string str) { 
    ParseString(str); 
  }

  void ParseString(string& str) {
     //...populare map
  }

  string ToString() { return m_MyString; }
};


class Derived : public Base {
public:
  Derived(string str) : Base(str) {};

  Derived(const Base& obj) {
    Base::ParseString( obj.ToString(); }
  }
};

I get an error: unable to cast 'this' from Base to Base& loss qualifier.

Another alternative is to write also an operator=() overload marking m_MyMap as protected...

Derived& operator=(const Base& obj) {
  if (this != &obj) {
    m_MyMap = obj.m_MyMap;
  }
}

As I said in my previous post, I need to construct (or assign) the Derived object from a (already) constructed Base object.

Cheers,
Daniele.

[mik2718]

What about just

Derived(const Base& obj) : Base(obj) { }

this uses the copy constructor in Base to initialize the base part