Re: Simple example on interfaces

["philip...@gmail.com" <philip.munts@gmail.com>]

> "IMHO, Interfaces are worthless." 

I find interfaces to be extremely valuable for abstracting I/O devices.  For example in my Linux Simple I/O Library, there is code equivalent to the following (the actual code is different, as I sucked a lot of common boilerplate for I/O device interfaces into a generic package that is instantiated for each data item type):

package GPIO is

  type Direction is (Input, Output);
  type PinInterface is interface;
  type Pin is access all PinInterface'Class;

  procedure Put(Self : PinInterface; state : Boolean);

  function Get(Self : PinInterface) return Boolean;
end GPIO;

I've probably defined a dozen packages that implement GPIO pins using everything from Linux kernel services to web servers.  Every one of them contains a function like this:

  function Create(...) return GPIO.Pin;

This allows code like the following:

  GPIO1 : GPIO.Pin := GPIO.libsimpleio.Create(RaspberryPi.GPIO18, GPIO.Output);
  GPIO2 : GPIO.Pin := GPIO.HTTP.Create("http://foo.munts.net", 5, GPIO.Output);
  GPIO3 : GPIO.Pin := GPIO.RemoteIO.Create(server, 7, GPIO.Output);

This allows GPIO pins scattered far and near throughout the known universe to be treated exactly the same, even collected into an array or container.

I very seldom implement more than one interface in a type definition though, unless a single device has multiple sensors (temperature and humidity, for instance).

Microsoft's .Net uses this scheme pervasively, though I originally learned it in Ada and later applied the same thinking to .Net, Free Pascal, Java, Python, and C++ (and other languages).