Re: Polymorphism and "interface_cast"
On 11/19/2010 09:03 PM, Thiago Adams wrote:
I was wondering why I have to express the interface relationships
on the object definition.
I don't have to express in the object type, for instance, if it
is allocated on the stack or on the heap then it can be used in both.
The same idea could be applied in polymorphism.
I could have the class definition and choose if I want it polymorphic
or not.
Even more, I want to choose how I will see the object, and it doesn't
need
to know about that.
One reason to do that is because the inheritance just complicated
design and
makes changes hard.
Static polymorphism and generic algorithms solve this problem in some
cases,
but they don't solve the problem when dynamic polymorphism is
required.
Let's say I have a list with Cars and Dogs. Both have the Color
property.
To apply an algorithm in this list, which uses the Color property, I
need
to implement a common Interface in Dogs and Cars.
I have to artificially create this common interface just to access the
Color.
If the base class of Dog is Animal and it already has the Color
property, this will not help,
because Car is not derived from Animal.
The same if the Car is derived from Vehicle and Vehicle has the Color
property.
Now let's say I have this Common interface but in another program I
will reuse only the Car class.
For this software, the Vehicle interface is enough, because I have a
polymorphic list of Vehicles.
I don't need to use the Animal class.
This sample shows that the same object Car can be "viewed" differently
in two places.
The Car object is exactly the same, it has the Color property. So the
"Beauty is in the eye of the beholder"
You have presented the wrong approach to the problem. This should never
be done this way.
Trying to address this question I did some experiment with an
"interface_cast"
Sample:
struct Car {
int Color() {return 1;}
};
struct Dog {
int Color() {return 2;}
};
I need in some software / algorithm see the Color of Car and Dog in a
polymorphic way.
For this, I will use this "view" or interface.
struct IColor { virtual int Color() = 0; };
The use of "interface_cast" is something like: (The final result)
Car car;
Dog dog;
IColor& rCar = interface_cast<IColor>(car);
cout<< rCar.Color();
IColor& rDog = interface_cast<IColor>(dog);
cout<< rDog.Color();
I believe that this issue should be generalized into
"how to simplify writing delegating functions and classes".
for example:
class InterfaceAdapter : public Interface {
public:
using Interface::auto (cat->auto);
private:
Cat * cat;
};
Now you could make all variants of interface_cast
in the library... and I would have a way to write
adapters more quickly.
--
Dragan
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