Why can't I use "is" on a class reference type?
lazarus / 62-General

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ahydra · 2017-03-11 15:40

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  TA =
class    ...  end;   TB = class(TA)    ...  end;   TAClass = class of
TA; function IsB(aType: TAClass): boolean;  begin    Result := (aType
is TB); // won't compile: Class or COM interface type expected, but
got "TAClass"end;  
(Lazarus 1.6.4, FPC 3.0.2)

Why is this not possible? Since all the "is" operator does is compare
the VMT pointer given by the RHS to the VMT pointer of the class
instance on the LHS (and recurse through its parents until there's a
match), it should also be possible to do the same thing with VMT
pointers in the form of class reference types.

Thanks,

ahydra

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Thaddy · 2017-03-11 15:49

A class reference is not a class.It is as simple as that.

Try:
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 function
IsB(aType: TAClass): boolean;  begin    Result := (aType =
TB.ClassType); // just compare the class referencesend;
A bit nicer is this (untested):
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 operator =
(const a:TClass;const b:Tobject) c:boolean;inline;begin  c :=
(a=b.classtype);end; operator i = (const a:Tobject;const b:TClass)
c:Boolean;inline;begin  c := (a.classtype = b);end;

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sky_khan · 2017-03-11 16:57

I think you're looking for "InheritsFrom",
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  
Result:=aType.InheritsFrom(TB);

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howardpc · 2017-03-11 16:59

Yes. In the following program, the function IsAClass(aClassRef:
TBClass) is pretty useless, since no TBClass (TB being declared as
other than TA) can ever be a TAClass, i.e. the outcome of the function
is always known in advance, so it hardly adds any useful information.

Potentially more useful is whether TA and TB are related, viz. the
DescendsFromA function. Note that (unlike your function IsB) the
parameter is a general TClass, which makes the applicabiity of the the
wrapped  InheritsFrom() function much wider than just to TAClass
classes.

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  TA = class end;  TB =
class(TA) end; TBClass = class of TB; function IsAClass(aClassRef:
TBClass): boolean;begin  Result:=(aClassRef =
TA.ClassType);end; function DescendsFromA(aClassRef: TClass):
Boolean;begin  Exit(aClassRef.InheritsFrom(TA));end; begin 
WriteLn('IsAClass(TB) is ',IsAClass(TB));  WriteLn('DescendsFromA(TB)
is ',DescendsFromA(TB));  ReadLn;end.

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ahydra · 2017-03-11 17:03

Fair enough - TB is a type rather an a VMT pointer, so you need to
call ClassType on it to get the VMT pointer. However Thaddy's solution
doesn't take care of inheritance (a class reference variable set to a
subclass of TB should also return true for "is TB").

Here's what I came up with:

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; uses sysutils; type  TA = class   end;   TB = class(TA)  
end;   TC = class(TB)   end;   TD = class(TA)   end;   TAClass = class
of TA; function ClassReferenceIs(aObjectClass, aTargetClass: TAClass):
boolean;  type    PVMT = ^TVMT;    TVMT = packed record      Useless:
ptruint;      Useless2: ptruint;      AncestorVMT: pointer;    end;  
var    p: pointer;   begin    p := pointer(aObjectClass);     repeat 
   if p = pointer(aTargetClass) then      begin        Result :=
true;        exit;      end;       p := PVMT(p)^.AncestorVMT;    until
p = nil;     Result := false;end; var  AC: TAClass; begin  AC := TA; 
writeln(booltostr(ClassReferenceIs(AC, TB), 'yep', 'nope')); // nope 
AC := TB;  writeln(booltostr(ClassReferenceIs(AC, TB), 'yep',
'nope')); // yep  AC := TC;  writeln(booltostr(ClassReferenceIs(AC,
TB), 'yep', 'nope')); // yep  AC := TD; 
writeln(booltostr(ClassReferenceIs(AC, TB), 'yep', 'nope')); // nope  
readln(); end. 
I think to be honest though, FPC should support "is" on class
references. It's a perfectly sensible thing to want to do, and can use
exactly the same algorithm as for class instances.

In fact, it should probably support things like "TB is TA" as well?
That may be rather useless, but it still makes sense - when compiling,
the type "TB" used as a value is just the VMT pointer of TB.

I see now two people have suggested InheritsFrom, that works
wonderfully - thanks.

ahydra