[fpc-pascal]fpc 1.0 and 1.1 incompatibility

Matt Emson memsom at interalpha.co.uk
Thu Oct 30 12:13:33 CET 2003

> fpc 1.1 gives the following error message:
> overloadedcontructor.pas(36,32) Error: Can't determine which overloaded
> function to call overloadedcontructor.pas(19,19) Hint: Found
> declaration: constructor TBase.Create(TComponent)
> overloadedcontructor.pas(26,23) Hint: Found declaration: constructor
> TSubClass.Create(TStrings) overloadedcontructor.pas(39) Fatal: There
> were 1 errors compiling module, stopping

In a way that makes a lot of sense... however it's a symptom of the way FPC
does overloading in objfpc mode. I've no idea why it's broken between 1.0
and 1.1, I can only think it's related to a missing patch?

Explanation: To the compiler, your declaration of the constructor is
'Create( Areference to object )' in both cases. The Object you pass is
typed, so the compiler will pick the overloaded constructor that fits the
type you are passing. It looks as if the logic concerning the presedence
that a constructor has over other overloaded constructors has changed. One
would assume that it used to be 'child class overrides the parent class',
hence 1.0 works. It seems that 1.1 takes all methods as equal, and passing
'nil' causes a paradox because nil is a possible value for both overloaded

In Delphi syntax, we have to explicitly define overloaded methods,

procedure x; overload;
procedure x(s: string); overload;

and this situation becomes clearer. I also believe the overloading rules are
tighter in Delphi and you wouldn't be able to do what you are doing anyway.

I suggest that your code needs to mark your constuctors as virtual, and that
your second class could override the first's constructor and call inherited
to pass the correct param. I don't see an advantage to passing completely
different classes as params to overloaded constructors otherwise.

Also remember that you can have constructors with different names (Create,
CreateStringList, etc) and so you could get around the issue that way.

Hope that elps,


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