BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Abstract constructor?          BorlandTalk.com Forum Index -> Delphi OO design View previous topic :: View next topic   Author Message Martin James Guest Posted: Wed Dec 15, 2004 10:00 am    Post subject: Abstract constructor? If I define a completely abstract class from TObject, eg: Tmailbox=class(Tobject) private function getCount: integer; virtual; abstract; protected public property count:integer read getCount; constructor create; virtual; abstract; procedure push(aObject:Tobject); virtual; abstract; function pop(pResObject:pObject;timeout:DWORD):boolean; virtual; abstract; destructor destroy; virtual; abstract; end; I cannot call any inherited create in my descendant classes. I have to have a constructor because the TObject constructor is static. I know that the TObject constructor does not do anything, but even so it looks wrong when I do not call any inherited create in the children. I can easily get round it by taking the'abstract' off the constructor and actually using a virtual constructor that calls 'inherited'. That looks right, but it seems like I am calling two constructors that do nothing :( Is there any other recommended design/bodge? Rgds, Martin PS. I was tempted to change my sig to 'Castro', but managed to resist :) Back to top Chris Trueman Guest Posted: Wed Dec 15, 2004 10:06 am    Post subject: Re: Abstract constructor? Maybe I'm missing something but why not define an interface with the Tmailbox methods and inherited from that? Chris. Back to top Joanna Carter (TeamB) Guest Posted: Wed Dec 15, 2004 10:14 am    Post subject: Re: Abstract constructor? "Chris Trueman" <ctrueman (AT) wavesoftware (DOT) com> a écrit dans le message de news: Xns95C0671CC4F2Dctruemanwavesoftware (AT) 207 (DOT) 105.83.66... Quote: Maybe I'm missing something but why not define an interface with the Tmailbox methods and inherited from that? Possibly because the OP doesn't understand how to manage the memory problems mixing objects and interfaces causes ? :-) Joanna -- Joanna Carter (TeamB) Consultant Software Engineer TeamBUG support for UK-BUG TeamMM support for ModelMaker Back to top Jim Cooper Guest Posted: Wed Dec 15, 2004 10:19 am    Post subject: Re: Abstract constructor? Quote: I cannot call any inherited create in my descendant classes. Well, no, you can't call TMailBox.Create because it's abstract Why not just leave it out altogether? Quote: I have to have a constructor because the TObject constructor is static. Can you explain that reasoning a bit? Cheers, Jim Cooper _______________________________________________ Jim Cooper [email]jim (AT) falafelsoft (DOT) com[/email] Falafel Software http://www.falafelsoft.com _______________________________________________ Back to top Martin James Guest Posted: Wed Dec 15, 2004 10:46 am    Post subject: Re: Abstract constructor? Quote: I cannot call any inherited create in my descendant classes. Well, no, you can't call TMailBox.Create because it's abstract Why not just leave it out altogether? I have to have a constructor because the TObject constructor is static. Can you explain that reasoning a bit? I cannot override a static method. Rgds, Martin Back to top Stefan Meisner Guest Posted: Wed Dec 15, 2004 11:02 am    Post subject: Re: Abstract constructor? Quote: I cannot override a static method. But you could declare the constructor as being virtual in your descending class, like TPersistent does. Greetings Stefan Back to top Jim Cooper Guest Posted: Wed Dec 15, 2004 11:20 am    Post subject: Re: Abstract constructor? Quote: I cannot override a static method. No, but I still don't see your problem. Why do you see a need for a constructor in your TMailbox class? Just introduce a constructor when you need to. Are you worried about polymorphic construction of objects? Cheers, Jim Cooper _______________________________________________ Jim Cooper [email]jim (AT) falafelsoft (DOT) com[/email] Falafel Software http://www.falafelsoft.com _______________________________________________ Back to top Bob Dawson Guest Posted: Wed Dec 15, 2004 12:34 pm    Post subject: Re: Abstract constructor? "Jim Cooper" wrote Quote: need to. Are you worried about polymorphic construction of objects? I assumed that as the intent, yes, so the code obj := TmailboxClass.Create; would function properly. Personally I'm not at all bothered by a virtual constructor that does nothing but call inherited, and I wouldn't optimize away from that standard code without very strong reason. Optimization at the program logic, domain model, and algorithm levels is generally much more important than optimizing individual code lines. bobD Back to top Jim Cooper Guest Posted: Wed Dec 15, 2004 3:54 pm    Post subject: Re: Abstract constructor? Quote: I assumed that as the intent, yes, so the code obj := TmailboxClass.Create; would function properly. But that'll function without a constructor defined in TMailBox at all :-) I don't have an issue with a one line virtual constructor either, but I'm not sure what the issue actually is Cheers, Jim Cooper _______________________________________________ Jim Cooper [email]jim (AT) falafelsoft (DOT) com[/email] Falafel Software http://www.falafelsoft.com _______________________________________________ Back to top Wayne Niddery [TeamB] Guest Posted: Wed Dec 15, 2004 4:10 pm    Post subject: Re: Abstract constructor? Martin James wrote: Quote: Tmailbox=class(Tobject) public constructor create; virtual; abstract; destructor destroy; virtual; abstract; end; I cannot call any inherited create in my descendant classes. I have to have a constructor because the TObject constructor is static. I know that the TObject constructor does not do anything, but even so it looks wrong when I do not call any inherited create in the children. I can easily get round it by taking the'abstract' off the constructor and actually using a virtual constructor that calls 'inherited'. That's the correct route. Quote: That looks right, but it seems like I am calling two constructors that do nothing Let the compiler/optimizer worry about that. A worse problem is your virtual spec on the destructor. Destroy is *already* virtual. You are destroying TObject polymorphism by redclaring this as virtual here. You should remove this altogether unless you need it to actually do something, and in that case, specify override, not virtual. -- Wayne Niddery - Logic Fundamentals, Inc. (www.logicfundamentals.com) RADBooks: http://www.logicfundamentals.com/RADBooks.html SpaceShipOne; GovernmentZero Back to top Bob Dawson Guest Posted: Wed Dec 15, 2004 4:56 pm    Post subject: Re: Abstract constructor? "Jim Cooper" wrote Quote: I assumed that as the intent, yes, so the code obj := TmailboxClass.Create; would function properly. But that'll function without a constructor defined in TMailBox at all Not if you're looking for a descendent object and TMailBox doesn't redefine the constructor as virtual. bobD Back to top Rob Kennedy Guest Posted: Wed Dec 15, 2004 5:41 pm    Post subject: Re: Abstract constructor? Bob Dawson wrote: Quote: "Jim Cooper" wrote I assumed that as the intent, yes, so the code obj := TmailboxClass.Create; would function properly. But that'll function without a constructor defined in TMailBox at all :-) Not if you're looking for a descendent object and TMailBox doesn't redefine the constructor as virtual. The constructor doesn't need to be virtual. Martin could use the following code, for instance. type TMailbox = class public procedure DoCreate; virtual; abstract; constructor Create; // not virtual end; TMailboxClass = class of TMailbox; constructor TMailbox.Create; begin inherited Create; DoCreate; end; He might not even need to use DoCreate at all. Maybe he could just override the AfterCreation method instead. And then TMailbox couldn't need a separate constructor, either. (If you're descending from TInterfacedObject, or you use a similar implementation for some other interfaced class, be careful about the order in which you call the inherited AfterConstruction method. That's the method that sets the object's reference count back to zero, so you mustn't allow any references to the object to be released after you call the inherited method.) -- Rob Back to top Jim Cooper Guest Posted: Wed Dec 15, 2004 5:43 pm    Post subject: Re: Abstract constructor? Quote: Not if you're looking for a descendent object and TMailBox doesn't redefine the constructor as virtual. Well, it depends. That's why I asked for clarification of the problem. I've using the Template Method pattern as per Rob's post before. Other times I've had a virtual constructor. Hard to give advice without knowing what the problem is :-) Cheers, Jim Cooper _______________________________________________ Jim Cooper [email]jim (AT) falafelsoft (DOT) com[/email] Falafel Software http://www.falafelsoft.com _______________________________________________ Back to top Cesar Romero Guest Posted: Sat Dec 18, 2004 2:59 pm    Post subject: Re: Abstract constructor? Quote: I cannot override a static method. I usually do reintroduce when I need a diferent constructor TBox = class(TObject) public constructor create(Mechanism: TCustomMechanism; Father: TObject); reintroduce; override; .... end; Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         BorlandTalk.com Forum Index -> Delphi OO design All times are GMT Page 1 of 1   Jump to: Select a forum C++ Builder Newsgroups----------------C++ BuilderC++ Builder (ActiveX)C++ Builder (Announcements)C++ Builder (Command Line Tools)C++ Builder DatabasesC++ Builder Databases (ADO)C++ Builder Databases (Desktop)C++ Builder Databases (InterBase Express)C++ Builder Databases (Multi-Tier)C++ Builder Databases (SQL Servers)C++ Builder (Graphics)C++ Builder (IDE)C++ Builder (Internet)C++ Builder (Internet Socket)C++ Builder (Internet Web)C++ Builder (Jobs)C++ Builder (Language)C++ Builder (Language C++)C++ Builder (Compatibility with MFC)C++ Builder (Native API)C++ Builder (Non-Technical)C++ Builder (Students)C++ Builder (Third Party Tools)C++ Builder (Upgrade)C++ Builder (VCL)C++ Builder (VCL Components Usage)C++ Builder (VCL Components Development)C++ Builder (VCL Usage)C++ Builder (WinAPI)C++ Builder Installation Delphi Newsgroups----------------Delphi (General)Delphi ActiveX Controls UsageDelphi ActiveX Controls DevelopmentDelphi As400Delphi DatabasesDelphi Databases (ADO)Delphi Databases (dbExpress)Delphi Databases (Desktop)Delphi Databases (InterBase Express)Delphi Databases (Multi-Tier)Delphi Databases (SQL Servers)Delphi DeploymentDelphi GraphicsDelphi IDEDelphi InternationalizationDelphi InternetDelphi Internet Isapi-WebBrokerDelphi Internet WebSnapDelphi Internet WinsockDelphi JobsDelphi Language BASMDelphi Language ObjectPascalDelphi Model Driven Architecture (General)Delphi Model Driven Architecture (Third Party)Delphi MultimediaDelphi Native APIDelphi Non-TechnicalDelphi ObjectPascalDelphi OLE AutomationDelphi OO designDelphi OpenToolsAPIDelphi Reporting-ChartingDelphi RTLDelphi StudentsDelphi TeamSourceDelphi Thirdparty ToolsDelphi Thirdparty Tools (General)Delphi Thirdparty Tools (IntraWeb)Delphi Thirdparty Tools (RaveReports)Delphi UpgradeDelphi VCLDelphi VCL ComponentsDelphi VCL Components UsingDelphi VCL Components WritingDelphi WebServices SOAPDelphi WinAPIDelphi XMLDelphi Installationcomp.lang.pascal.delphi.components.misccomp.lang.pascal.delphi.components.usagecomp.lang.pascal.delphi.components.writingcomp.lang.pascal.delphi.databasescomp.lang.pascal.delphi.misccomp.sources.delphi JBuilder Newsgroups----------------JBuilderJBuilder AnnouncementsJBuilder Applet IssuesJBuilder As400JBuilder CompilerJBuilder CORBA/RMIJBuilder DatabasesJBuilder DB SwingJBuilder DebuggerJBuilder DeploymentJBuilder DocumentationJBuilder EnterpriseJBuilder HandheldJBuilder IDEJBuilder JavaJBuilder JavaAPIJBuilder Java LanguageJBuilder Java SwingJBuilder Java BeansJBuilder Java Beans UsageJBuilder Java Beans DevelopmentJBuilder JBCLJBuilder JDataStoreJBuilder JobsJBuilder LinuxJBuilder MacintoshJBuilder & MobileSetJBuilder MultiLingual AppsJBuilder Non-TechnicalJBuilder OpenToolsJBuilder Servlets (JSP)JBuilder StudentsJBuilder Team DevelopmentJBuilder Thirdparty ToolsJBuilder UpgradeJBuilder Installation InterBase Newsgroups----------------InterBaseInterBase Bug DiscussionInterBase GeneralInterBase IBConsoleInterBase OpenSourceInterBase ReplicationInterBase SQLInterBase ThirdpartyInterBase Installation Miscellaneous Borland Newsgroups----------------AppCenterAppServerEnteraVisiBroker  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.