Problem using Interfce

[H. S. Teoh]

On Mon, May 14, 2012 at 12:15:32PM -0700, Ali Çehreli wrote:
> On 05/14/2012 11:40 AM, Stephen Jones wrote:
> > I want an array of different classes of objects.
> 
> So far, Object[] makes sense.
> 
> > I tried to subsume the differences by extending the classes under a
> > single interface/abstract class/super class (3 different approaches)
> > all to no avail
> 
> Yeah, that is a fundamental example of object oriented design. Thank
> you for showing us some code below. It help a lot.
> 
> > as I could not access the public variables of the instantiated
> > classes while storing them in an array defined by the
> > interface/super class.
> 
> Makes sense because neither the interface nor the super class has
> such members. D's polymorphism does not have virtual values (nor
> does C++'s, the other object oriented language that I know well).
> Polymorphism is about virtual member functions.

D's @property attribute does allow you to simulate virtual data members,
though:

	class Base {
		private int _width, _height;
		this() { _width = 1; _height = 1; }
		@property int width() { return _width; }
		@property int height() { return _height; }
	}

	class Derived : Base {
		// Look, ma! .width is a virtual value!
		override int width() { return 0; }
	}

	void main() {
		auto obj1 = new Base;
		assert(obj1.width == 1);

		auto obj2 = new Derived;
		assert(obj2.width == 0);
	}


> What you can do is to force the subclasses provide the data through
> virtual functions.

You can also downcast. But downcasting generally should be avoided
because it breaks the Liskov Substitution Principle. It's better to
provide virtual functions to access derived class data.


> (An alternative is to use compile-time polymorphism through
> templates. They provide data virtualization. (See range interfaces
> like InputRange. For example, the return type of front() is not
> specified by InputRange. Ranges can be of any type of elements.))
[...]

Templates are instantiated at compile-time, though. It won't work if
what you put in the list changes at runtime.


T

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