Hi,
I'm trying to use template in fpc 2.6.0. I can map class type as value as above code can be compiled. But if I uncomment the declaration of tIS -- map class type as key, fpc fail to compile and print out these error msgs:

Error: Operator is not overloaded: "TObject" < "TObject" Error: Operator is not overloaded: "TObject" > "TObject" t.pas(10) Fatal: There were 2 errors compiling module, stopping Fatal: Compilation aborted
Error: /usr/bin/ppcx64 returned an error exitcode (normal if you did not specify a source file to be compiled)

Any way to resolve it?

It fails because the operators < and > are not defined for TObject, which is assumed by TFPGMap<TKey, TValue>.KeyCompare().

As a makeshift you can implement a record type to wrap TObject and define these operators for it. See the attached file for an example.

Hope it helps.

The OP asks for generics usage, not for the operator overload (notice the 'specialize' keyword).

L.

Notice the {$MODE DELPHI} directive.

Thanks for your makeshift :) . But the problem still confusing me. My code is in type-section and it's a declaration not a statement.

tFpGMap<tObject

The form of the code is similar to a comparision expression and `<' here looks like a less-than operator. But they are not. In my opinion, fpc should not take the declaration as a statement then try to search for the nonsense operator-overloaded methods.

I’m not quite getting what you’re trying to say… Free Pascal and Delphi do have distinct syntaxes for generic type definition and specialization. You may want to consult respective documentations to clarify your understanding of the code.

◦ Delphi ―
<http://docwiki.embarcadero.com/RADStudio/en/Overview_of_Generics> ◦ Free Pascal ―
<http://www.freepascal.org/docs-html/ref/refch8.html>

Hope it helps.

above code can be compiled. But if I uncomment the declaration of tIS -- map class type as key, fpc fail to compile and print out these error msgs:

specify a source file to be compiled)

Any way to resolve it?

This has nothing to do with < and > in the declaration. TFPGMap uses < and > comparisons for key comparisons, so these operators need to be overloaded. You can't use global operators here, because of scope problems, so you need to use either a different type or wrap them in a record with operators.

Regards,
Sven

2012/4/26 ZHANG Dao-yuan :

You might want to have a look at
http://code.google.com/p/fprb/wiki/TheCmpBTreeMap

-jan

Yes, I didn't clearly understand the generic mechanism in fpc/tfpgmap. I've read, `A class can be seen as a pointer to an object, or a pointer to a record, with methods associated with it.' in fpc reference.pdf, chapter 6 - Classes. Hence I think fpc takes object as pointer when specialize tfpgmap template.

Now I catch your idea. The specialization of tfpgmap is similar to std::map in C++ (must define a (bool operator<(ref to another obj)const) function for the KEY class). I'll read the source code of unit fgl to deepen understanding.

Thank you very much.

I have another question now. Why doesn't the developers of fgl overload `>' `<' operator for class-type in fgl unit? ie, put

operator >(a, b: tObject) r: boolean; operator <(a, b: tObject) r: boolean;

into the interface section
and put

into the implementation section.

This seems a easy way to resolve the scope problem.

ZHANG Dao-yuan wrote

I have another question now. Why doesn't the developers of fgl overload `>' `<' operator for class-type in fgl unit?

Ah, it will override overloaded-class-(lessThan|greaterThan)-operator which is defined in other units. :( By the way, Will `class operator' within class be allowed to define in the next major release?