Assigning interface

[egsuh]

Hi,  I have defined interfaces like following.

Code: Pascal  [Select][+][-]

1. unit Interface1
2. interface
3. type
4.      IDBAccess = interface
5.           //...
6.      end;
7. var
8.      DBAccess: IDBAccess;
9. implementation
10. // ...
11. end.
12.  
13. unit dModule1
14. interface
15. uses Interface1;
16. type
17.      Tdm1 = class (TDataModule, IDBAccess)
18.          .....
19.      end;
20. implementation
21. procedure Tdm1.DataModuleCreate(Sender:TObject);
22. begin
23.      IDBAccess := Self;
24. end;
25. // ...
26. end.

But this calls SIGSEV error when closing the application. I've read somewhere that interfaces do reference counting and when there are no objects it try to destroy implementing object. Does the error occur because of either interface or object try to destroy the object which was destroyed already by the other?

Or, does my way of using interface have serious problem? I did not have any problem when I declared interface field (i.e. DBAccess here) within class.

[marcov]

The cardinal rule of interfaces is that typically you create the class and immediately assign it to an interface and never mix variables that are class references with variables that are interface references on the same object.

As such that is probably not very compatible with the VCL that is mostly class reference based.

[PascalDragon]

The cardinal rule of interfaces is that typically you create the class and immediately assign it to an interface and never mix variables that are class references with variables that are interface references on the same object.

As such that is probably not very compatible with the VCL that is mostly class reference based.

Any descendant of TComponent (which includes TDataModule) by default reimplements _AddRef and _Release to not do any reference counting (except if VCLComObject is assigned).

@egsuh: please provide a full example that shows your problem and not merely snippets.

[egsuh]

]@egsuh: please provide a full example that shows your problem and not merely snippets.

I tried to write a small project replicating the exceptions but they failed. My original project have dozens of units so it's not practical to post it here. I'll search for some reason myself and post it if I find the reason or other relevant issue.

[ccrause]

Code: Pascal  [Select][+][-]

1. type
2.      Tdm1 = class (TDataModule, IDBAccess)
3.          .....
4.      end;

Are you perhaps setting the AOwner parameter when creating your TDataModule? This could result in the AOwner also freeing the TDataModule instance.

[egsuh]

Are you perhaps setting the AOwner parameter when creating your TDataModule? This could result in the AOwner also freeing the TDataModule instance.

That's what I have thought. TDataModule is auto-created by application. But I could not replicate the exception with a small project.

I reviewed my original program again, and I found the definitions are :

Code: Pascal  [Select][+][-]

1. interface
2. type
3.      IICommon = interface
4.           // ....
5.      end;
6.  
7.      IDBAccess = interface(IICommon)
8.           //...
9.      end;
10. var
11.      DBAccess: IDBAccess;
12. implementation
13. // ...
14. end.
15.  
16. unit dModule1
17. interface
18. uses Interface1;
19. type
20.      Tdm1 = class (TDataModule, IICommon, IDBAccess)
21.          .....
22.      end;

There are vars of both types of IICommon and IDBAccess.

The exception does not occur if I write both of :

Code: Pascal  [Select][+][-]

1. procedure Tdm1.DataModuleCreate(Sender:TObject);
2. begin
3.      IDBAccess := Self;
4. end;
5.  
6. procedure Tdm1.DataModuleDestroy(Sender:TObject);
7. begin
8.      IDBAccess := nil;
9. end;
10.