[fpc-pascal] FPImage and GetDataLineStart

[Michael Van Canneyt]

On Thu, 21 Apr 2011, Sven Barth wrote:

> On 21.04.2011 21:45, Michael Van Canneyt wrote:
>> 
>> 
>> On Thu, 21 Apr 2011, Marco van de Voort wrote:
>> 
>>> In our previous episode, Michael Van Canneyt said:
>>>> 
>>>> You can propose whatever you want _but_ generics.
>>> 
>>> Motivation?
>> 
>> Threefold:
>> a) The compiler's handling of generics is still beta code in my opinion.
>> As far as I know, the original problem I reported when writing the docs
>> for them is still not solved.
>> 
>
> May I ask out of curiosity which problem you mean?

There were several.

The first one seems solved (see docs, section on 'A word on scope', the local thing), 
compiler now gives an error (a debatable solution IMHO, but better than the crash at the time.).

Second problem:

The following compiles:

uses ucomplex;

type
   Generic TMyClass<T> = Class(TObject)
     Function Add(A,B : T) : T;
   end;


Function TMyClass.Add(A,B : T) : T;

begin
   Result:=A+B;
end;


Type
   TMyIntegerClass = specialize TMyClass<Integer>;
   TMyComplexClass = specialize TMyClass<Complex>;

end.

but the following does not:

---------------------------------------------------------
unit mya;

interface

type
   Generic TMyClass<T> = Class(TObject)
     Function Add(A,B : T) : T;
   end;

Implementation

Function TMyClass.Add(A,B : T) : T;

begin
   Result:=A+B;
end;

end.
---------------------------------------------------------

and program :

---------------------------------------------------------
program myb;

uses mya, ucomplex;

Type
   TMyIntegerClass = specialize TMyClass<Integer>;
   TMyComplexClass = specialize TMyClass<Complex>;

end.
---------------------------------------------------------

Results in:

home: >fpc -S2 myb.pp
mya.pp(16,12) Error: Operator is not overloaded: "complex" + "complex"
myb.pp(9,14) Fatal: There were 1 errors compiling module, stopping

Which is strange to say the least, as the per the definition/intent of generics, 
the code in mya should not know anything about the types that will be used when 
specializing. (in this case complex).

As per the intent of generics the second program above should compile just as well.

But then different rules will apply for operators and procedure calls:
- procedure calls must be resolvable at define time
- Operators must be resolvable at specialization time.
No principal problem (we can define it so), but strange at least.

(BTW. The sources of the docs contain a section about this second problem, 
but it is commented out in the LaTeX sources)

Michael.