BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Can two separate instances of a class share the same instanc          BorlandTalk.com Forum Index -> Delphi OO design View previous topic :: View next topic   Author Message Abdullah Kauchali Guest Posted: Fri Feb 04, 2005 7:55 am    Post subject: Can two separate instances of a class share the same instanc Is it possible to create two instances of descendant classes but have them both share one instance of a parent class? I'm obviously missing something basic here. TParent Quote: TChild child1 := TChild.Create(); child2 := TChild.Create(); //<-- creates another instance of TParent TParent *cannot* be a singleton. We can have many instances of TParent. Is it somehow possible to pass a pre-instantiated TParent into the create method of TChild so that it uses that as its ancestor instead of creating a new one? Thanks is advance & kind regards, Abdullah Back to top Joanna Carter (TeamB) Guest Posted: Fri Feb 04, 2005 8:34 am    Post subject: Re: Can two separate instances of a class share the same ins "Abdullah Kauchali" <non (AT) non (DOT) com> a écrit dans le message de news: 42032a61$1 (AT) newsgroups (DOT) borland.com... Quote: Is it possible to create two instances of descendant classes but have them both share one instance of a parent class? Don't create the Parent inside the Child, create it first and then pass it to the child constructor. parent := TParent.Create; child1 := TChild.Create(parent); child2 := TChild.Create(parent); Joanna -- Joanna Carter (TeamB) Consultant Software Engineer TeamBUG support for UK-BUG TeamMM support for ModelMaker Back to top Abdullah Kauchali Guest Posted: Fri Feb 04, 2005 9:00 am    Post subject: Re: Can two separate instances of a class share the same ins Joanna Carter (TeamB) wrote: Quote: Don't create the Parent inside the Child, create it first and then pass it to the child constructor. parent := TParent.Create; child1 := TChild.Create(parent); child2 := TChild.Create(parent); Hi Joanna, thanks for assisting ... Won't the inheritance structure automatically create a new instance of Parent each time TChild.create is executed? Regards Abdullah .... ps. I am posting the "real" question ... what I'd like to model via proper Class structures ... my OO knowledge is poor. Back to top John Leavey Guest Posted: Fri Feb 04, 2005 9:05 am    Post subject: Re: Can two separate instances of a class share the same ins Abdullah Kauchali wrote: Quote: Is it possible to create two instances of descendant classes but have them both share one instance of a parent class? I'm obviously missing something basic here. TParent | TChild child1 := TChild.Create(); child2 := TChild.Create(); //<-- creates another instance of TParent TParent cannot be a singleton. We can have many instances of TParent. Is it somehow possible to pass a pre-instantiated TParent into the create method of TChild so that it uses that as its ancestor instead of creating a new one? Thanks is advance & kind regards, Abdullah Either :- Make the contructor of TChild take a parameter of type TParent so that the appropriate Parent value is assigned to an internal field of the new TChild instance TParent = Class; TChild = Class private FParent; public constructor Create( P: TParent ); end; ... constructor TChild.Create( P: TParent ); begin inherited Create; FParent := P end; or Add a CreateChild() function to the TParent class that returns a new instance of TChild - this routine assigns Self to the Parent field of the new TChild instance TParent = Class; TChild = Class; private FParent: TParent; end; TParent = Class public function CreateChild: TChild; end; ... function TParent.CreateChild; begin Result := TChild.Create; Result.FParent := Self; end; John Leavey Back to top Abdullah Kauchali Guest Posted: Fri Feb 04, 2005 9:07 am    Post subject: Re: Can two separate instances of a class share the same ins Thanks John, Let me chew on this please ... gimme a moment to construct the example. .... I'll be back ... Back to top Joanna Carter (TeamB) Guest Posted: Fri Feb 04, 2005 9:13 am    Post subject: Re: Can two separate instances of a class share the same ins "Abdullah Kauchali" <non (AT) non (DOT) com> a écrit dans le message de news: [email]42033a02 (AT) newsgroups (DOT) borland.com[/email]... Quote: Won't the inheritance structure automatically create a new instance of Parent each time TChild.create is executed? Ah, now I see that you mean Parent as being the ancestor class. No, you cannot separate the instantiation of the child class from the instantiation of the parent class. The child class is said to have an 'is a' relationship; using parent and child is not a good example for this explanation, so let me use an anlogy here : Person ^ Quote: Student A Student 'is a' Person; you cannot have two Students who are the same Person. When we use the terms Parent and Child, we usually use that to refer to the 'has a' concept of Aggregation or Composition, where one object either contains or owns other objects. Is this the concept you are trying to design ? Joanna -- Joanna Carter (TeamB) Consultant Software Engineer TeamBUG support for UK-BUG TeamMM support for ModelMaker Back to top Joanna Carter (TeamB) Guest Posted: Fri Feb 04, 2005 9:17 am    Post subject: Re: Can two separate instances of a class share the same ins "John Leavey" <johnl@[no_spam].compufile.co.uk> a écrit dans le message de news: [email]42033aee (AT) newsgroups (DOT) borland.com[/email]... Quote: Make the contructor of TChild take a parameter of type TParent so that the appropriate Parent value is assigned to an internal field of the new TChild instance Nice try John, but Abdullah has already replied that he is trying to separate the instantiation of the ancestor (parent) class from the derived (child) class. I have replied, asking for clarification of his situation; please add your comments as the discussion progresses. Joanna -- Joanna Carter (TeamB) Consultant Software Engineer TeamBUG support for UK-BUG TeamMM support for ModelMaker Back to top Abdullah Kauchali Guest Posted: Fri Feb 04, 2005 9:20 am    Post subject: Re: Can two separate instances of a class share the same ins Joanna Carter (TeamB) wrote: Quote: When we use the terms Parent and Child, we usually use that to refer to the 'has a' concept of Aggregation or Composition, where one object either contains or owns other objects. Beautiful. I am seeing a speck of light already! ... Quote: Is this the concept you are trying to design ? Yes! How do we go about aggregating the data (fields) of different instances of an ancestor class so that they are one and only one version of the truth for multiple instances of the descendants? (I hope I phrased the question correctly this time!) Back to top Joanna Carter (TeamB) Guest Posted: Fri Feb 04, 2005 10:48 am    Post subject: Re: Can two separate instances of a class share the same ins "Abdullah Kauchali" <non (AT) non (DOT) com> a écrit dans le message de news: [email]42033e7a (AT) newsgroups (DOT) borland.com[/email]... Quote: Yes! How do we go about aggregating the data (fields) of different instances of an ancestor class so that they are one and only one version of the truth for multiple instances of the descendants? (I hope I phrased the question correctly this time!) Not quite. We will have to take this a step at a time ) Aggregation and Composition have nothing to do with Inheritance. You talk about ancestors and descendants, but these are terms that are only used in inheritance. Let me define more strongly Aggregation and Composition : Aggregation - One object contains a list of other objects. - The contained objects can exist without being part of the containing object - example : Car -> Wheel - you can take the Wheels and put them on another Car before destroying the original Car Composition - One object contains a list of other objects - The contained object has no context outside of the containing object - example : Invoice -> Invoice Line - an Invoice Line can only belong to one Invoice, if you delete the invoice, the lines go with it. Does your model fit into either of these ? If not then could you give real names to the classes you are trying to model ? Joanna -- Joanna Carter (TeamB) Consultant Software Engineer TeamBUG support for UK-BUG TeamMM support for ModelMaker Back to top Rudy Velthuis [TeamB] Guest Posted: Fri Feb 04, 2005 12:08 pm    Post subject: Re: Can two separate instances of a class share the same ins Abdullah Kauchali wrote: Quote: Won't the inheritance structure automatically create a new instance of Parent each time TChild.create is executed? Why? -- Rudy Velthuis [TeamB] http://rvelthuis.bei.t-online.de "I think there is a world market for maybe five computers." -- Thomas Watson (1874-1956), Chairman of IBM, 1943 Back to top Abdullah Kauchali Guest Posted: Fri Feb 04, 2005 12:10 pm    Post subject: Re: Can two separate instances of a class share the same ins Rudy Velthuis [TeamB] wrote: Quote: Abdullah Kauchali wrote: Won't the inheritance structure automatically create a new instance of Parent each time TChild.create is executed? Why? Bad terminology ... Joanna corrected me. It was supposed to be phrased thusly: Won't the inheritance structure automatically create a new instance of the *ancestor* each time the *descendant* is created? Back to top Rudy Velthuis [TeamB] Guest Posted: Fri Feb 04, 2005 12:15 pm    Post subject: Re: Can two separate instances of a class share the same ins Abdullah Kauchali wrote: Quote: Bad terminology ... Joanna corrected me. It was supposed to be phrased thusly: Won't the inheritance structure automatically create a new instance of the ancestor each time the descendant is created? You mean as part of the descendant instance? Yes, of course. To avoid this, you should not use inheritance. -- Rudy Velthuis [TeamB] http://rvelthuis.bei.t-online.de "Some cause happiness wherever they go; others, whenever they go." - Oscar Wilde (1854-1900) Back to top Abdullah Kauchali Guest Posted: Fri Feb 04, 2005 12:18 pm    Post subject: Re: Can two separate instances of a class share the same ins Rudy Velthuis [TeamB] wrote: Quote: You mean as part of the descendant instance? Yes, of course. To avoid this, you should not use inheritance. Correcto! :) 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.