[fpc-pascal] Implementing a true Singleton - Can we decrease the visibility of a method?

[Jonas Maebe]

On 8 dec 2006, at 14:03, Graeme Geldenhuys wrote:

> There is a lot of things in life we shouldn't do, but we do.  :-)
> At least give us the choice, maybe via a compiler directive and keep
> the compiler warning in place.  That way we can use it responsibly
> when required,

This is a generic argument which you can use in support of pretty  
much any relaxing of semantic/syntax rules.

> as in the case of implementing a true singleton. I'm
> sure there are more examples. As it stands currently (my sample
> singleton code) the developer can still screw up by creating a new
> instance instead of going through the global function.

This particular issue is caused by the fact that in Delphi-style  
Object Pascal classes automatically inherit from TObject, which has a  
public constructor. And if you override the constructor with your  
own, they no longer can do that either (although it's obviously a run  
time and not compile time check).

One thing that could be changed is the compile so that you get a  
compile-time error if you try

type
   tc = class
     constructor create; virtual; abstract;
   end;

var
   c: tc;
begin
   c := tc.create
end.

(currently this compiles with only a warning about the fact that tc  
has abstract methods)


It would however still only give a run time error if you'd instead do

type
   tc = class
     constructor create; virtual; abstract;
   end;

  ttc = class of tc;

var
   cc: ttc;
   c: tc;
begin
   cc := tc;
   c := cc.create;
end.


Jonas