Re: [fpc-pascal] generic class helper and class references

[Sven Barth]

On 29.09.2013 05:30, Xiangrong Fang wrote:

type
   generic TTreeT = class
 ... ...
   end;
   TTreeType = class of TTree;


This is illegal code and compiled by pre-2.7.1 FPC only because of bugs 
in the generic implementation. TTree is not a legal type outside of 
the generic's declaration or implementation (and no, TTreeT is neither).



1) is it possible to define class helper for generic classes?


No. I recently had the thought to allow this as well, but not now.


2) why class helper require a class and we cannot use a class reference?


A class helper always work on variables (or return values) of the type 
you specified. So if you'd be able to define a class helper for 
TTreeType you'd only be able to use it on variables of type TTreeType or 
e.g. TTreeType(Self.ClassType), but not on instances of TTree.


Regards,
Sven
___
fpc-pascal maillist  -  fpc-pascal@lists.freepascal.org
http://lists.freepascal.org/mailman/listinfo/fpc-pascal

[fpc-pascal] generic class helper and class references

[Xiangrong Fang]

Hi There,

While playing with generics (ref. class inheritance and type
incompatibility), I have an idea to use generic class helper to overcome
the problem that generic methods cannot be virtual (because the compiler
see specialized class different than the generic one?)

I have a TTree generic class (ref:
https://github.com/xrfang/fpcollection/blob/master/src/units/tree.pas),
which currently has problem with the Clone method, I did the following:

type
  generic TTreeT = class
... ...
  end;
  TTreeType = class of TTree;
  TTreeHelper = class helper for TTreeType;

The compiler does not compile because it thinks you cannot use a class
reference (TTreeType) while defining a class helper. Why? Shouldn't class
references has same effect as a Class?

Ok, since it does not compile, I changed the last line to the following
alternatives, but NONE of them compiles:

1) TTreeHelper = class helper for TTree;
2) generic TTreeHelperT = class helper for TTreeT;

My purpose is that since the Clone method cannot be virtual (because
descendant class will not have same return type as the generic class), I
try to write a class helper for ALL specialized TTree and their descendants.

Of course I can (and will) follow Sven's advice in that email to modify my
TTree class. This email is just two other questions:

1) is it possible to define class helper for generic classes?
2) why class helper require a class and we cannot use a class reference?

Best Regards,
Xiangrong
___
fpc-pascal maillist  -  fpc-pascal@lists.freepascal.org
http://lists.freepascal.org/mailman/listinfo/fpc-pascal