Class Reference Type Parameter With Default Value
lazarus / 62-General

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simone · 2020-02-10 23:27

I'm surely missing some basic thing… I apologize in advance… Let
suppose we want to use a procedure with a class reference type
parameter having a default value, as in the following example:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i program
Project1;{$mode ObjFpc}type  C=class  end;   C1=class(C)  end;  
C2=class(C1)  end;   Cl = class of C;   procedure Proc(aCl : Cl=C1);
//<-- illegal expression  begin  end; beginend. 
Why does the compiler says that there is an illegal expression in the
definition of Proc? Thanks.

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PascalDragon · 2020-02-11 08:23

You can't. Only those values that are also allowed for untyped
constants (ordinals, floating point values, strings, sets) are allowed
as default values.

You need to solve this using an overload:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
Proc(aCl: Cl);beginend; procedure Proc; inline;begin  Proc(C1);end;
If you place the implementation of the overload without the parameter
after the one with the parameter and the former has the inline
modifier (like in the example above; for a unit you need to add the
modifier to the declaration in the interface section) then you'll
essentially have the same as only one call will be involved then.

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simone · 2020-02-11 08:31

Thanks PascalDragon.  Waiting for the forum reply I had used this
workaround (but without the intelligent use of the inline modifier). 
I didn't know this limitation of parameters with default values. 
Thanks for the explanation.

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simone · 2020-02-11 12:40

Quote from: PascalDragon on February 11, 2020, 09:23:47 amOnly those
values that are also allowed for untyped constants (ordinals, floating
point values, strings, sets) are allowed as default values.

As a side note:  It's possible to set a class reference type parameter
with NIL as default value:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i program
Project1;{$mode ObjFpc}type  C=class  end;   C1=class(C)  end;  
C2=class(C1)  end;   Cl = class of C;   procedure Proc(aCl : Cl=nil);
//<-- OK  begin  end; beginend.

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jamie · 2020-02-11 21:40

Your first attempt failed because you tried to change the type to
another type, even if it is the same. You can only define a default
value not a default type..

C1 here is a type not a variable.

The second attempted work because NIL is a value not a type which
equals a zero pointer.

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simone · 2020-02-11 23:28

Thanks Jamie. Indeed the concept of class reference in Object Pascal
is similar, but different, respect to the notion of metaclass (i.e.
class whose instances are classes) available in other OOP languages. I
was misled by this analogy...

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PascalDragon · 2020-02-12 08:24

Quote from: jamie on February 11, 2020, 10:40:32 pmYour first attempt
failed because you tried to change the type to another type, even if
it is the same. You can only define a default value not a default
type..

C1 here is a type not a variable.

The second attempted work because NIL is a value not a type which
equals a zero pointer.

Nonsense. For class types a class type is a valid value:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i type 
TMyObject = class  end;   TMyClass = class of TMyObject;  
TMySubObject = class(TMyObject); var  MyClass: TMyClass =
TMySubObject;
This is one of the outstanding features of Object Pascal.

It's only that default parameters have further restrictions that
simone's code does not work.

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jamie · 2020-02-12 21:49

Of course you can do that but he was doing it within the parameter
case call.

different state all together..

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PascalDragon · 2020-02-13 08:23

That's what I already said: that default parameters have further
restrictions. But what you wrote was more general:

Quote from: jamie on February 11, 2020, 10:40:32 pmYou can only define
a default value not a default type..

To someone skimming over the posts it might not be clear that you're
talking strictly about default parameters, thus my clarification.