Re: data and interfaces

Mr Fish wrote: 
howdi,

I'm curious to how people feel about this... I often create an
interface then realize that *all* subclasses will have similar data
such as, say, a list of floats so I define the interface as

class ISomeInterface
{
protected:

  list<float> m_Floats;

public:

  ~ ISomeInterface();

  virtual void SomeMethod()=0;

};


I'm sure though that the purists would exclude the data from the
intertrface class and I'm sure there must be some good reasons to do
so.

Can you provide me with any? (assuming that the data type will not
change from subclass to subclass *ever*)

The above is the reason.... we can not - or rather should not assume anything about the Implementation of the Interface.

The main aim of Interfaces is to separate the interface (aka the Type) from the implementation (aka the class that 'is a' Type).

Don't think of the implementors of Interfaces as subclasses - thats a C++ implementation detail. Classes 'implement' an Interface - they don't extend them.


If you decide that all current implementors of the Interface have commonality (like the float list) then you have a number of choices.

1) Create an Abstract Base Class and put the list there. Then each class that 'implements' the interface can also derives from the ABC.

2) Create an Abstract Adapter class - its a class that in C++ terms derives from the Interface, but still does not implement the interface methods. This adapter can then have the list of floats.

Either of these ways are maintaining the separation of Implementation from Interface, allowing other implementors of the interface to not be burdened with unneeded extras.

Andrew

BTW - there's nothing 'purist' about this.
.

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