BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  One TMyComponent per project          BorlandTalk.com Forum Index -> C++ Builder (VCL Components Development) View previous topic :: View next topic   Author Message JD Guest Posted: Fri Jul 02, 2004 9:17 pm    Post subject: One TMyComponent per project I'd like to be able to code the component's constructor to automatically check if it's the only instance of it's class in the project when it's dropped on the form. If it isn't, I want it to say something and then delete itself. I'd also like to force it to be dropped on the main form if possible. ~ JD Back to top Remy Lebeau (TeamB) Guest Posted: Fri Jul 02, 2004 10:57 pm    Post subject: Re: One TMyComponent per project "JD" <nospam (AT) nospam (DOT) com> wrote Quote: I'd like to be able to code the component's constructor to automatically check if it's the only instance of it's class in the project when it's dropped on the form. The easiest way to do that is with a global pointer that is assigned to the first instance of your component. Then additional instances can check to see if the pointer is already assigned or not. For example: --- MyComponent.cpp --- TMyComponent *Instance = NULL; __fastcall TMyComponent::TMyComponent(TComponent *Owner) : TMyBase(Owner) { if( Instance ) throw Exception("Only one instance can be created at a time!"); Instance = this; } __fastcall TMyComponent::~TMyComponent() { Instance = NULL; } Quote: If it isn't, I want it to say something and then delete itself. Simply throw an exception, as shown above. Everything else will be handled for you. Quote: I'd also like to force it to be dropped on the main form if possible. That, on the other hand, is a little harder to manage. In the constructor, the only thing you have access to regarding the creator of the component instance is the Owner parameter. When you drop a component onto a form, the form is assigned as the Owner, so you could do the following: __fastcall TMyComponent::TMyComponent(TComponent *Owner) : TMyBase(Owner) { if( Instance ) throw Exception("Only one instance can be created at a time!"); if( Owner ) { if( GetParentForm(Owner) != Application->MainForm ) throw Exception("Can only be placed on the MainForm!"); } Instance = this; } The trouble comes when the component is being instantiated dynamically at runtime. The calling code would be able to specify any Owner it wants to, which may not be the MainForm at all. And you can't use the Parent property to decide because the Parent has not been assigned yet while you are still inside the constructor. Gambit Back to top JD Guest Posted: Sat Jul 03, 2004 6:34 am    Post subject: Re: One TMyComponent per project "Remy Lebeau (TeamB)" <gambit47.no.spam (AT) no (DOT) spam.yahoo.com> wrote: Quote: [...] Simply throw an exception, as shown above. Everything else will be handled for you. That's easy enough. Quote: I'd also like to force it to be dropped on the main form if possible. [...] When you drop a component onto a form, the form is assigned as the Owner, [...] The trouble comes when the component is being instantiated dynamically at runtime. Your sample gave me the idea that I could use dynamic_cast to test the Owner as a TForm prior to testing if it's the main form: if( !dynamic_cast<TForm*>(Owner) ) { // then we know for sure that it's being dynamically allocated throw Exception("You must assign the MainForm as the Owner."); } if( GetParentForm(Owner) != Application->MainForm ) { // can't tell if it's dynamically allocated throw Exception("Can only be placed on the MainForm!"); } Knowing if it's dynamically allocated or not is only important in as much as providing an exact reason for throwing. I'll just have to settle for a more generic message: if( !dynamic_cast<TForm*>(Owner) || GetParentForm(Owner) != Application->MainForm ) { throw Exception("The Owner must be the MainForm!"); } Thanks. ~ JD Back to top Remy Lebeau (TeamB) Guest Posted: Sat Jul 03, 2004 8:31 am    Post subject: Re: One TMyComponent per project "JD" <nospam (AT) nospam (DOT) com> wrote Quote: Your sample gave me the idea that I could use dynamic_cast to test the Owner as a TForm prior to testing if it's the main form: The only problem with that code is that GetParentForm() does not validate whether the specified component is NULL, or whether it is already a form. It always looks at the component's Parent immediately. Thus, your code logic should be more like the following instead: TCustomForm *form = NULL; if( Owner ) { form = dynamic_cast<TCustomForm*>(Owner); if( !form ) form = GetParentForm(Owner); } if( (!form) || (form != Application->MainForm) ) throw Exception("You must assign the MainForm as the Owner."); Gambit Back to top JD Guest Posted: Sat Jul 03, 2004 10:05 am    Post subject: Re: One TMyComponent per project "Remy Lebeau (TeamB)" <gambit47.no.spam (AT) no (DOT) spam.yahoo.com> wrote: Quote: [...] Thus, your code logic should be more like the following instead: I'm too tired right now to understand what was wrong with my code. I'll have to look at it later but thanks for the sample. ~ JD Back to top Michael Harris Guest Posted: Sat Jul 03, 2004 12:53 pm    Post subject: Re: One TMyComponent per project Hard code the component name property afterconstruction ? "JD" <nospam (AT) nospam (DOT) com> wrote Quote: I'd like to be able to code the component's constructor to automatically check if it's the only instance of it's class in the project when it's dropped on the form. If it isn't, I want it to say something and then delete itself. I'd also like to force it to be dropped on the main form if possible. ~ JD Back to top Remy Lebeau (TeamB) Guest Posted: Sat Jul 03, 2004 9:33 pm    Post subject: Re: One TMyComponent per project "JD" <nospam (AT) nospam (DOT) com> wrote Quote: I'm too tired right now to understand what was wrong with my code. Ignore it. My mistake, I misread how GetParentForm() works, and thought that it does not check the specified Control itself to see if it is a form. It actually does, so your earlier code is fine, although I would suggest getting rid of dynamic_cast: if( !Owner ) throw Exception("You must assign an Owner."); if( GetParentForm(Owner) != Application->MainForm ) throw Exception("Can only be placed on the MainForm!"); Gambit Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         BorlandTalk.com Forum Index -> C++ Builder (VCL Components Development) 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.