[fpc-devel] Circular references or Forward Declarations

[Jonas Maebe]

On 14 Aug 2009, at 08:43, Graeme Geldenhuys wrote:

> I can't agree more. Sometimes there is a valid design for something  
> like that. The compilers already handle forward declarations in the  
> same unit,

Actually, only in the same "type" block.

> so why can that not be applied for multiple units as well.

Because types with the same name in different units are different  
types in Pascal.

Suchfunctionality will probably be added for Objective-Pascal classes  
(in the form of external anonymous objcclass types, because otherwise  
it's very hard to make sensible automated translations of Objective-C  
framework headers to Pascal; single giant umbrella units don't work  
very well there), but there it can work because the type system of  
Objective-C works differently. It has one namespace for class names,  
which is global for the entire application because all classes have to  
be registered with the run time system based on their name. So in  
Objective-C, a class name by itself does refer to a unique type (and  
the compiler will give an error if you try to define a class with the  
same name twice in different units).

In regular Object-Pascal, class names do not have to be unique over  
the entire application. Letting the compiler guess which type is the  
external one you meant and which isn't is a recipe for disaster. It  
would probably also require a significant rewrite of the type handling  
and symbol table (even more so than what will be required for objcpas).


Jonas