Can I make the compiler fail if Create is called explicitly?

I have many singletons where I don't want the developers to call the constructor Create. Instead they should use the function Instance

TdgpBankBranch = class(TDataGroup)
strict private
  class var InstanceVar : TdgpBankBranch;
private
  { Only for internal use }
  constructor Create (AOwner : TComponent); reintroduce;
protected
public
  class function Instance : TdgpBankBranch;
  class function Is_A_Live_Singleton: boolean;
  property Rec[Bank, Branch : integer]: TBankBranch read GetRec; default;
end;

I have been shifting the constructors to private. This makes the code analysers complain "Non-public constructor".

I could shift them back to public. Is there a way to make the compiler fail if an attempt is made at using the constructor ?

Solution

You ask how to stop the compiler from creating an object when somebody calls .create, but what you're trying to do is create a singleton, so it helps to see how others solved this.

In Delphi, singletons are often implemented via a function that's declared in the interface section and which returns an object that's declared in the implementation section of a unit. The function creates the instance if it doesn't exist yet, to provide lazy loading. Cleanup is done via the finalization section.

In your situation, instead of having an instance() function, you just create a global function that gives you an instance.

This approach probably stems from the days before classes and private sections existed, but it's pretty straightforward, and prevents the issue that you're running in to. Look at how TClipboard is implemented. This implementation is not possible in Java or C# because they don't have global functions, or the type of scoping that makes this Delphi implementation work...

Or take a look at this answer by David Heffernan: Delphi Singleton Pattern

It's even better than TClipboard, because it hides the entire class in the implementation section, truly preventing people to create instances themselves, but you'll have to create an interface for your class.