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Subject: Re: Simple example on interfaces
From: "philip...@gmail.com" <philip.munts@gmail.com>
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Xref: reader02.eternal-september.org comp.lang.ada:61215
List-Id: <comp.lang.ada>

> "IMHO, Interfaces are worthless."=20

I find interfaces to be extremely valuable for abstracting I/O devices.  Fo=
r example in my Linux Simple I/O Library, there is code equivalent to the f=
ollowing (the actual code is different, as I sucked a lot of common boilerp=
late 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 every=
thing from Linux kernel services to web servers.  Every one of them contain=
s a function like this:

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

This allows code like the following:

  GPIO1 : GPIO.Pin :=3D GPIO.libsimpleio.Create(RaspberryPi.GPIO18, GPIO.Ou=
tput);
  GPIO2 : GPIO.Pin :=3D GPIO.HTTP.Create("http://foo.munts.net", 5, GPIO.Ou=
tput);
  GPIO3 : GPIO.Pin :=3D 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, fo=
r 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, P=
ython, and C++ (and other languages).