Polymorphism and virtual setters

[fzuend]

Hi,

I'm new to c++ (but I know other languages) and I'm confused about how to implement something. I hope one of you can solve this (probably very simple) problem:

My goal is to write this somewhere in the code:

Car *mycar = new GCar();
myCar->SetParam(new GParam());

To do this, I created the following classes:

class Car
{
   virtual void SetParam(Param *aa) = 0;   
};

class GCar: public Car
{
	virtual void  SetParam(GParam *aa) {a = aa;} //Overrides the SetParam from Car, right?
	GParam *a;
};

class Param
{
};

class GParam: public Param
{
};

However, I get a 'cannot instantiate abstract class' error on this line myCar->SetParam(new GParam());. I don't get it, can anybody help?

Thanks!

Fabio

[tummychow]

Your error is because Car contains a pure virtual. Any class containing a pure virtual is abstract and no objects of it (including pointers) may be declared.

[PanGalactic]

class GCar: public Car
{
	virtual SetParam(GParam *aa) {a = aa;} //Overrides the SetParam from Car, right?
	GParam *a;
};

You are not overriding the virtual method because the method signature changed. The parameter is now GParam, but the virtual method was defined with Param.

[fzuend]

Thanks for the hints! So Car should not contain any pure virtual methods? How should I design the classes in order to be able to write

Car *mycar = new GCar();
myCar->SetParam(new GParam());

[tummychow]

As PanGalactic noted, functions are only virtual if they have the same name *and* argument list. So you'd need GCar::SetParam to take a Param, or Car::SetParam to take a GParam. Alternatively, you could create a wrapper that encapsulates this inheritance hierarchy, or you could set up a virtual in the Param hierarchy that handles how SetParam responds when it takes one.
Like I said, if you want to declare any object of Car you need to make Car::setparam a non-pure function. It's that simple.

[R0mai]

fzuend wrote:

Thanks for the hints! So Car should not contain any pure virtual methods? How should I design the classes in order to be able to write

So, if I understand correctly, you want to SetParam() behave differently if the object is a GCar and the paramter is GParam?
I believe, your class hierarchy is broken somewhere. Could you post more information about what is the purpose of Param and GParam?

tummychow wrote:

Any class containing a pure virtual is abstract and no objects of it (including pointers) may be declared.

You can declare pointers and references of classes which contain pure virtual methods.

struct Base {
    virtual void f() = 0;
};

struct Derived : Base {
    virtual void f() {}
};

int main() {
    Base *b = new Derived;
}

[fzuend]

Eventually I'd like to write something like:

Car *c;
if (blup)
  c = new GCar();
  c->SetParams(new GParams(1,2,3,4));
else if (blap)
  c = new FCar();
  c->SetParams(new FParams(1,2,3,4,5,6));
[...]

doCrazyStuff(c);

And, just now, I found a solution and it's ridiculously simple:

class Car
{   
};

class GCar: public Car
{
	void SetParam(GParam *aa) {a = aa;} 
	GParam *a;
};

class Param
{
};

class GParam: public Param
{
};

ect.

and then I use it as follows:

Car *c;
if (blup)
  GCar *gc = new GCar();
  gc->SetParams(new GParams(1,2,3,4));
  c = gc;
else if (blap)
  FCar *fc = new FCar();
  fc->SetParams(new FParams(1,2,3,4,5,6));
  c = fc;
[...]

doCrazyStuff(c);

[fzuend]

The solution I was initially looking for was generics!

Something like

class Car <T: Param>{...};

class GCar: Car <GParam>{...};