Interfaces

From the viewpoint of memory management, interfaces in Delphi are implemented as classes with added reference counting. To create an interface, you actually have to create an object of a class, which will get a reference count of 1. If you then assign this interface to another variable, both will point to the same memory and the reference count will be incremented to 2.

There is no equivalent to SetLength or UniqueString that would make a unique copy of an interface. That would require duplicating the underlying object and Delphi has no built-in support for that.

The object implementing the interface is destroyed when its reference count falls to 0:

var
  i1, i2: IInterface;
begin
  i1 := TInterfacedObject.Create;
  // i1 points to an object with reference count 1
  i2 := i1;
  // both i1 and i2 point to a same object with reference count 2
  i1 := nil;
  // i2 now points to an object with reference count 1
  i2 := nil;
  // reference count dropped to 0 and object is destroyed
end;

Although interfaces are very similar to classes, all this reference count management takes its cost. It is implemented with something called interlocked instructions which are a bit slower than normal increment/decrement instructions. I'll discuss this in more detail in Chapter 5, Getting Started with the Parallel World.

This only makes a measurable difference when you are assigning interfaces a lot, but sometimes this is exactly what happens. I'll show an example in the next section.