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

Vincent Snijders vsnijders at quicknet.nl
Fri Dec 8 10:31:22 CET 2006

Graeme Geldenhuys schreef:
> On 12/8/06, Vincent Snijders <vsnijders at quicknet.nl> wrote:
>> Maybe I am analyzing your output wrong, but to me it seems as if the 
>> contructor of
>> TObject is called in the third case. printdata seems to print an empty 
>> text.
>> So the protected contructor is unreachable, but the public constructor 
>> in TObject
>> remains public.
> That's correct, which is even worse.  It gives us a half-baked object.
> fData is never initialized.  Putting a writeln() statement inside
> TSingleton.Create shows that on the 3rd instance it is never executed.

I think the solution is to create a public constructor which throws an 
NotImplemented exception or something like that.

Then you create a protected (or private) constructor
constructor CreateSingletonInstance;
which is accessible only in that unit.

Note: instead of making function GetSingleton : TSingleton; a global function you 
could make it class function GetInstance too.


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