Question

Just discovered something rather funny:

var
  Queue  : TQueue <TProc>;
  MyProc : TProc;
...
MyProc := Queue.Dequeue;

I think you see what is intendend here. However, the compiler thinks I want to store the Queue.Dequeue method (type “procedure of object”) in MyProc and reports an error

E2010 Incompatible Types: 'TProc' und 'Procedure of object'

The workaround I came up with goes like this

MyProc := TProc (Pointer (Queue.Dequeue));

Is there a more elegant solution?

Answer 1

There’s a bit of syntactical ambiguity there about whether the name “Dequeue” refers to the function itself, or the function’s return value. And since you’re dealing with an anonymous method pointer which you can assign a normal function to, it’s trying to interpret that as a function assignment, not a function result assignment. Casting it to a pointer is the wrong solution, as that would force the function assignment to go through, which would then cause all sorts of fun errors when you attempt to invoke MyProc.

The correct way to fix it is by removing the syntactical ambiguity. Put an empty parenthesis after Dequeue, so that the compiler is sure that you’re calling the function and not simply referencing it by name, and then it’ll work.

MyProc := Queue.Dequeue();