CODEWITHSUNDEEP
delphiinterfacereference
david
david

Reputation: 2638

delphi object lifetime, interface re-assignment (simple question)

It is well known (???) that Delphi reference-counts interface references to interfaced objects, and destroys the objects when their reference counts drop to zero, normally because all relevant interface variables fall out of scope.

Suppose I use a global interface variable to refer to a persistent object:

type 
  tMySyncClass = class(TInterfacedObject,TmyInterface); ...  
  tMyAsyncClass = class(TInterfacedObject,TmyInterface); ...

var MyInterfaceVar : TmyInterface;

procedure MyProc();
begin
  MyInterfaceVar := tMySyncClass.create();
  ....
  MyInterfaceVar := tMyAsyncClass.create();
end

After calling MyProc, I have an instance of tMyAsyncClass, with one (global) MyInterface reference to it.

Do I still have an unreferenced MySyncClass object? Does re-assigning MyInterfaceVar trigger destruction of the first (SyncClass) object?

Upvotes: 1

Views: 130

Answers (1)

Remy Lebeau
Remy Lebeau

Reputation: 595305

Assigning an interfaced object to an interface variable will increment the refcount of the object being assigned, and decrement the refcount of the object the variable was previously referring to.

So yes, re-assigning the MyInterfaceVar variable will remove the reference to the tMySyncClass object, decrementing its refcount and thus freeing the object since it has no more active references.

Upvotes: 5

Related Questions