GPC and CodeWarrior Object Pascal

15 Apr 2006

      Dear all,

while porting from CodeWarrior Object Pascal to GPC I noticed the 
following differences which might be worthwhile to address in GPC.

1) Mutual recursive object type declaration
program prog1; { compile with gpc --mac-pascal prog1.pas }
type
     t1 = object
         function f: t2; { error: unknown identifier `t2' }
{ above works in CodeWarrior Object Pascal as objects are always 
references }
     end;

     t2 = object
         function g: t1;
     end;

function t1.f: t2; begin f := nil; end;
function t2.g: t1; begin g := nil; end;
begin
end.

As a workaround this can be solved by introducing a common super class 
and subsequent casting.

2) Procedure type assignment compatibility with specialized parameters
program prog2; { compile with gpc --mac-pascal prog2.pas }
type
     baseType = object
     end;

     derivedType = object(baseType)
     end;

procedure p1(x: baseType); begin end;
procedure p2(x: derivedType); begin end;
var pv: procedure(x: baseType);
begin
     pv := p1;
     pv := p2; { error: incompatible types in assignment }
end.

A workaround is to declare the x in p2 as baseType and perform a cast in 
the body of p2.

3) Cast in with statement
program prog3; { compile with gpc --mac-pascal prog3.pas }
type
     baseType = object
     end;

     derivedType = object(baseType)
         procedure f;
     end;

procedure derivedType.f; begin end;

var a: baseType;
     b: derivedType;

begin
new(b);
a := b;
derivedType(a).f; { ok }
with derivedType(a) do
{ warning: `with' element is not a constant or an lvalue    }
     f;
{ error: invalid lvalue in method call                      }
{ error: function call used as a statement                  }
end.

A workaround is to use a temporary variable for the with statement.

Peter Schorn

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