[fpc-pascal] A better way?

Tony Whyman tony.whyman at mccallumwhyman.com
Thu Apr 14 10:02:37 CEST 2016


Reading through your post, I hope that you are aware that most circular 
dependencies can be avoided by referencing other units from the "uses" 
clause in the "implementation" section and keeping the "uses" clauses in 
"interfaces" down to those references strictly necessary for the unit's 

Having probably told you something you already know, i do sometimes get 
circular reference problems and usually because a program has evolved 
rather than having been designed properly. Sometimes it's better just to 
take as a hint to structure your program better but, otherwise, the 
techniques available are:

1. Abstract classes (as you suggest)

2. Interfaces

3. Dynamic casts.

Of the three, dynamic casts are often the quick and dirty way of fixing 
the problem as they can allow you to move a problematic unit reference 
from the "interface uses" to the "implementation uses", replacing the 
class reference in the unit interface to something generic like 
"TObject" and them coercing it to the required class when you actually 
use it.

I hope this helps.


Tony Whyman

On 14/04/16 05:29, Ryan Joseph wrote:
> The most annoying problem I have with Pascal currently is with circular unit dependanices and “global” classes that need to be referenced from many units. In other languages I would make a forward declaration of the class in  one file then implement it in another file so that all files could reference the class. It’s maybe a symptom of a “bad" design but sometimes it’s just faster and easier so it’s a problem I have to fight Pascal to make it work.
> When I moved to FPC the solution I started using was this pattern below where I make an abstract class then override the methods I need in the global namespace within in the “main unit”. This is a bad hack to workaround a feature of the language but I wonder if there’s a better way. Does anyone have any ideas I could try?
> ========
> "global” unit shared by many other units:
> type
>    TSomeClassAbstract = class (TObject)
>      procedure DoSomething; virtual; abstract;
>    end;
> “main” unit which implements the class:
> type
>    TSomeClass = class (TSomeClassAbstract)
>      procedure DoSomething; override;
>    end;
> Regards,
> 	Ryan Joseph
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