BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Component Design Goto page 1, 2  Next          BorlandTalk.com Forum Index -> Delphi OO design View previous topic :: View next topic   Author Message Chris Trueman Guest Posted: Tue Jan 04, 2005 9:54 pm    Post subject: Component Design I've built a component that essentially manages a list of objects. In this case I chose to return a new object each time an item in the list was retrieved as in: property Item[ix: Integer]: TService read GetItem; function TServiceList.GetItem(ix: Integer): TService; var s: PEnumServiceStatus; begin if (ix < 0) or (ix >= FCount) then Result := nil else begin { FItems is just a block of memory containing records } s := FItems; while ix > 0 do begin Inc(s); Dec(ix); end; { Calls a private constructor that picks apart the record } Result := TService.Create(s); end; end; This feels like a design that runs contrary to most list management components I've seen. Thoughts? Chris. Back to top Peter Morris [Air Softwar Guest Posted: Tue Jan 04, 2005 10:12 pm    Post subject: Re: Component Design Quote: This feels like a design that runs contrary to most list management components I've seen. Thoughts? If you were lazy-creating the item and keeping the reference in a list to pass back the next time it was requested then I wouldn't have an objection, but that is not what you are doing. One of my golden-never-break rules is to *never* return a new object from a function, if you do you are at high risk of introducing memory leaks, for example. if list.item[0] <> nil then list.item[0].Caption := 'Hello'; You have just created two different objects, neither of which have been freed. -- Pete ==== Audio compression components, DIB graphics controls, FastStrings http://www.droopyeyes.com Read or write articles on just about anything http://www.HowToDoThings.com My blog http://blogs.slcdug.org/petermorris/ Back to top Chris Trueman Guest Posted: Tue Jan 04, 2005 10:21 pm    Post subject: Re: Component Design Quote: if list.item[0] <> nil then list.item[0].Caption := 'Hello'; This is a good observation. Since the TServicesManager.Services also returns a new object each time - code like: for iii := 0 to svrMgr.Services.Count - 1 do if svrMgr.Services[iii].ServiceName = 'blah blah' presumbely creates the list object Count + 1 times. Thanks for the comments. Chris. Back to top Bryan Crotaz Guest Posted: Wed Jan 05, 2005 12:47 pm    Post subject: Re: Component Design Quote: One of my golden-never-break rules is to *never* return a new object from a function, if you do you are at high risk of introducing memory leaks, for example. Actually it's fine, as long as your method is named well Eg CreateItem: TItem or NewItem: TItem. Bryan Back to top Jim Cooper Guest Posted: Wed Jan 05, 2005 2:56 pm    Post subject: Re: Component Design Quote: One of my golden-never-break rules is to *never* return a new object from a function How do you do factory patterns then? :-) 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 Jan 05, 2005 3:07 pm    Post subject: Re: Component Design "Jim Cooper" wrote Quote: How do you do factory patterns then? I'm also a bit more relaxed about this, and try to mitigate things via coding conventions: factory classes are always called T<~>Factory, and other classes the have object-returning functions tend to name those functions Create<~> or Provide<~> rather than just Get<~>. This tells the programmer that the function assumes no responsibily for the provided object, whereas the result of an Item[x] or GetItem(x) call can safely be walked away from. bobD Back to top Derek Davidson Guest Posted: Wed Jan 05, 2005 3:27 pm    Post subject: Re: Component Design Peter Morris [Air Software Ltd] wrote: Quote: One of my golden-never-break rules is to never return a new object from a function Whoa! That's a severe, self-imposed limitation there. If everyone followed the rule, here are a few places where significant re-writes would have to occur: 1. The VCL 2. The FCL 3. Refactoring - Improving the Design of Existing Code 4. Design Patterns I think, on balance, I'll keep the option open <g> -- Derek Davidson http://www.ebsms.com Send SMS Text messages from your PC. For FREE! Back to top Charles McAllister Guest Posted: Wed Jan 05, 2005 3:34 pm    Post subject: Re: Component Design Peter Morris [Air Software Ltd] wrote: Quote: One of my golden-never-break rules is to *never* return a new object from a function, if you do you are at high risk of introducing memory leaks, for example. if list.item[0] <> nil then list.item[0].Caption := 'Hello'; You have just created two different objects, neither of which have been freed. How about just following a naming convention, e.g. CreateItem? or DoCreateItem (looks less like a constructor call). if i saw... if list.CreateItem[0] <> nil then list.CreateItem[0].Caption := 'Hello'; ....be easier to see the red flag i also follow a naming convention of TryCreateXXX for cases where a function might return nil if object could not be created. Back to top Peter Morris [Air Softwar Guest Posted: Wed Jan 05, 2005 5:37 pm    Post subject: Re: Component Design Quote: Actually it's fine, as long as your method is named well Eg CreateItem: TItem or NewItem: TItem. I'd prefer procedure TMyClass.CreateItem(var NewItem: TItem); That way it is 100% impossible to cause this problem. Back to top Peter Morris [Air Softwar Guest Posted: Wed Jan 05, 2005 5:44 pm    Post subject: Re: Component Design Quote: How do you do factory patterns then? I always create a procedure with a "var" parameter, like so ............CreateWhatever(var Result: TWhatever) This way it is enforced that there must be a variable to hold the instance reference. The only time I would have a function returning a new object is when there is a parent object which holds a reference to it automatically, such as Person.Children.AddNew -- Pete ==== Audio compression components, DIB graphics controls, FastStrings http://www.droopyeyes.com Read or write articles on just about anything http://www.HowToDoThings.com My blog http://blogs.slcdug.org/petermorris/ Back to top Joanna Carter (TeamB) Guest Posted: Wed Jan 05, 2005 5:54 pm    Post subject: Re: Component Design "Peter Morris [Air Software Ltd]" <peter.morris (AT) remove (DOT) this.airsoftware.co.uk> a écrit dans le message de news: 41dc2786$1 (AT) newsgroups (DOT) borland.com... Quote: I always create a procedure with a "var" parameter, like so ...........CreateWhatever(var Result: TWhatever) This way it is enforced that there must be a variable to hold the instance reference. The only time I would have a function returning a new object is when there is a parent object which holds a reference to it automatically, such as In that case, you might like to consider using the 'out' modifier as this guarantees that you get nil if it fails, rather than accidentally passing in a valid object and getting the same object back instead of a replacement ?? Joanna -- Joanna Carter (TeamB) Consultant Software Engineer TeamBUG support for UK-BUG TeamMM support for ModelMaker Back to top Bryan Crotaz Guest Posted: Wed Jan 05, 2005 5:55 pm    Post subject: Re: Component Design "Peter Morris [Air Software Ltd]" <peter.morris (AT) remove (DOT) this.airsoftware.co.uk> wrote Quote: Actually it's fine, as long as your method is named well Eg CreateItem: TItem or NewItem: TItem. I'd prefer procedure TMyClass.CreateItem(var NewItem: TItem); That way it is 100% impossible to cause this problem. That's horrible to use though. I do a lot of this: ProcessHandler.Create(MyObject.CreateClone); or MyProcess.DataObject := MyObject.CreateClone; The Process owns the object, so all is hunkydory. Bryan Back to top Derek Davidson Guest Posted: Wed Jan 05, 2005 6:13 pm    Post subject: Re: Component Design Chris Trueman wrote: Quote: In this case I chose to return a new object each time an item in the list was retrieved as in: In the first case, I'm assuming that you're using Delphi (Win32) as this is a 'freeing the memory' issue and with .NET you get GC (and hence no issue) If you have control over the objects that you return how about returning an object that implements interfaces and thus get the objects reference counted and freed automatically? -- Derek Davidson http://www.ebsms.com Send SMS Text messages from your PC. For FREE! Back to top Bob Dawson Guest Posted: Wed Jan 05, 2005 6:17 pm    Post subject: Re: Component Design "Bryan Crotaz" wrote Quote: ProcessHandler.Create(MyObject.CreateClone); hmmm--rather the opposite of Peter's original case (you're passing an unreferenced new object to a function rather than out of one). If ProcessHandler.Create fails, what happens to MyObject? Rather than IntendedOwner.AddItem(TSomeObject.Create); -- what happens if the AddItem fails? I tend to do things like var sobj : TSomeObject; begin sobj := TSomeObject.Create; try IntendedOwner.AddItem(sobj); except sobj.Free; raise; end; bobD Back to top Jim Cooper Guest Posted: Wed Jan 05, 2005 6:32 pm    Post subject: Re: Component Design Quote: I always create a procedure with a "var" parameter, like so I find that a bit of an issue with factories, since you cannot have allocated Result yet. Dealing with failure gets a bit ugly. (I'm also not keen on Result as a parameter name , but that's a spearate problem) Cheers, Jim Cooper _______________________________________________ Jim Cooper [email]jim (AT) falafelsoft (DOT) com[/email] Falafel Software http://www.falafelsoft.com _______________________________________________ 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 Goto page 1, 2  Next Page 1 of 2   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.