What are the pros and cons of using interfaces in Delphi?

Question

I have used Delphi classes for a while now but never really got into using interfaces. I already have read a bit about them but want to learn more.

I would like to hear

Answer 1

Extra note on Cons: Performance

I think many people are too blithely dismissing the performance penalty of interfaces. (Not that I don't like and use interfaces but you should be aware of what you are getting into). Interfaces can be expensive not just for the _AddRef / _Release hit (even if you are just returning -1) but also that properties are REQUIRED to have a Get method. In my experience, most properties in a class have direct access for the read accessor (e.g., propery Prop1: Integer read FProp1 write SetProp1). Changing that direct, no penalty access to a function call can be significant hit on your speed (especially when you start adding 10s of property calls inside a loop.

For example, a simple loop using a class

for i := 0 to 99 do
begin
  j := (MyClass.Prop1 + MyClass.Prop2 + MyClass.Prop3) / MyClass.Prop4;
  MyClass.Update;
  // do something with j
end;

goes from 0 function calls to 400 function calls when the class becomes an interface. Add more properties in that loop and it quickly gets worse.

The _AddRef / _Release penalty you can ameliorate with some tips (I am sure there are other tips. This is off the top of my head):

• Use WITH or assign to a temp variable to only incur the penalty of one _AddRef / _Release per code block
• Always pass interfaces using const keyword into a function (otherwise, you get an extra _AddRef / _Release occurs every time that function is called.