micja491
asked on
Circular reference problem
Hello, I have a problem with developing an elegant object oriented application in Delphi 6.
For example I need two classes TSalesperson and TCustomer where each instance of a salesperson needs to know what customers it is responsible for and each customer-instance need to know what salesperson they belong to.
The natural thing would be to do the following:
unit uSalesperson;
implementation
uses
uCustomer;
type
TSalesperson = class
private
FCustomer: TCustomer;
end;
========================== ========== ======
unit uCustomer;
implementation
uses
uSalesperson;
type
TCustomer = class
private
FSalesperson: TSalesperson;
end;
When I do this and have each class residing in it's own unit I obviously get a circular reference warning from the compiler.
I can think of two workarounds for this problem:
1. Put both classes in the same unit
2. Make the fields of typ TObject and cast the references to the correct classes at runtime, in that case i could make one unit use(uses) the other in the implementation-section instead of in the interface and the compiler would be happy.
But I don't like either of these solutions because they will give me problems later on.
so my question is:
Is there any other way to solve this problem, perhaps by using interfaces or anything else? I'd be happy for any ideas.
Thanks
Mike
For example I need two classes TSalesperson and TCustomer where each instance of a salesperson needs to know what customers it is responsible for and each customer-instance need to know what salesperson they belong to.
The natural thing would be to do the following:
unit uSalesperson;
implementation
uses
uCustomer;
type
TSalesperson = class
private
FCustomer: TCustomer;
end;
==========================
unit uCustomer;
implementation
uses
uSalesperson;
type
TCustomer = class
private
FSalesperson: TSalesperson;
end;
When I do this and have each class residing in it's own unit I obviously get a circular reference warning from the compiler.
I can think of two workarounds for this problem:
1. Put both classes in the same unit
2. Make the fields of typ TObject and cast the references to the correct classes at runtime, in that case i could make one unit use(uses) the other in the implementation-section instead of in the interface and the compiler would be happy.
But I don't like either of these solutions because they will give me problems later on.
so my question is:
Is there any other way to solve this problem, perhaps by using interfaces or anything else? I'd be happy for any ideas.
Thanks
Mike
Hi micja491,
I suppose that there is no problems with warning, this is not error, right? It is standard way to let one unit know about another placing USES clause bellow IMPLEMENTATION part. So, why you care about warnings?
------
Igor.
I suppose that there is no problems with warning, this is not error, right? It is standard way to let one unit know about another placing USES clause bellow IMPLEMENTATION part. So, why you care about warnings?
------
Igor.
ASKER CERTIFIED SOLUTION
membership
This solution is only available to members.
To access this solution, you must be a member of Experts Exchange.
>>ITugay: In D5 it is an error no warning.
do you mean that it will produce error in D5?
//------------------------ ------
unit Unit2;
interface
implementation
uses
Unit3;
end.
//------------------------ ------
unit Unit3;
interface
implementation
uses
Unit2;
end.
btw, looking and Delphi's VCL source, you can notice that all classes which has cross-references, located in the same unit.
I would do by the same way, e.g:
type
TCustomer = class;
TSalesPerson = class;
TCustomer = class(TObject)
FSalesPerson: TSalesPerson;
end;
TSalesPerson = class(TObject)
FCustomer: TCustomer;
end;
-----
Igor.
do you mean that it will produce error in D5?
//------------------------
unit Unit2;
interface
implementation
uses
Unit3;
end.
//------------------------
unit Unit3;
interface
implementation
uses
Unit2;
end.
btw, looking and Delphi's VCL source, you can notice that all classes which has cross-references, located in the same unit.
I would do by the same way, e.g:
type
TCustomer = class;
TSalesPerson = class;
TCustomer = class(TObject)
FSalesPerson: TSalesPerson;
end;
TSalesPerson = class(TObject)
FCustomer: TCustomer;
end;
-----
Igor.
ASKER
Thanks all of you for your comments:
Cecario:
The forward declaration solution you recommended
Type
TSalesperson = Class;
TCustomer = Class;
doesn't work as Jacco pointed out (unless of course you put the Forward declarations in the same unit and section as the full class declarations, but then we're back to square one.
ITugay:
You are right that if you put the uses-clauses in the implementation part of the unit then you don't get the circular reference error. But for two classes that need to reference each other I have to have the class declarations in the interface-section, Otherwise i won't be able to set the correct type of the reference-variables right?
And about putting all reference-classes in the same unit, it will make my code messy, since most of my major classes will need to reference each other so most of the apps code will be in one unit making it hard to navigate.
Jacco:
So far I think that your suggestion with the Interfaces is the neatest one. I don't really mind the extra work of creating interfaces but then I'd have to put all interfaces in the same Unit, but that's quite a bit less messy since the Interfaces contain no impementation code.
So if no-one has any other suggestions I'll try this out and accept this comment in a couple of days.
/Mike
Cecario:
The forward declaration solution you recommended
Type
TSalesperson = Class;
TCustomer = Class;
doesn't work as Jacco pointed out (unless of course you put the Forward declarations in the same unit and section as the full class declarations, but then we're back to square one.
ITugay:
You are right that if you put the uses-clauses in the implementation part of the unit then you don't get the circular reference error. But for two classes that need to reference each other I have to have the class declarations in the interface-section, Otherwise i won't be able to set the correct type of the reference-variables right?
And about putting all reference-classes in the same unit, it will make my code messy, since most of my major classes will need to reference each other so most of the apps code will be in one unit making it hard to navigate.
Jacco:
So far I think that your suggestion with the Interfaces is the neatest one. I don't really mind the extra work of creating interfaces but then I'd have to put all interfaces in the same Unit, but that's quite a bit less messy since the Interfaces contain no impementation code.
So if no-one has any other suggestions I'll try this out and accept this comment in a couple of days.
/Mike
Add a new Unit ( UNIT0.PAS );
Unit UNIT0.PAS;
Interface
uses Objects,Classes;
Type
TSalesperson = Class;
TCustomer = Class;
Implentation
End.
Now add the Unit0 to Unit1 and Unit2 and it works
good luck
Cesario