[fpc-devel] Circular references and forward declarations

[Jonas Maebe]

On 05 Jan 2010, at 17:07, Juha Manninen wrote:

> Forward declaration of a class is allowed inside a unit. There can  
> be a
> reference to a class before it is defined! Like:
>  TMyClass = class;
>
> Why is it not allowed from another unit? They are conceptually the  
> same thing!

The reason that they are conceptually not the same thing is that in  
Pascal two different units can both declare a different class with the  
same name (just like they can both have global variables and  
procedures/functions in the interface with the same name). You would  
at least have to do something like one of the following
a) use one global name space for all classes (i.e., forbid that two  
different units used in a program declare a class with the same name  
anywhere), or
b) add some way to specify the unit name in which this external class  
is specified, or

There might be other solutions (maybe some kind of class-specific  
namespace support), but it would definitely require some more language  
features rather than merely accepting anonymous class definitions  
anywhere. Otherwise the type checking is going to run completely  
haywire.


Jonas