BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Using classes          BorlandTalk.com Forum Index -> C++ Builder (VCL Components Usage) View previous topic :: View next topic   Author Message Russell Hinds Guest Posted: Mon Dec 22, 2003 3:12 pm    Post subject: Using classes Can someone show me how to do the following, i am trying to learn about classes, i know its like a struct but a bit different. How do you write a class, and initialize the class variables to null when you first call the class variables. Lets use this simple struct for example. struct TNamesInfo { AnsiString Name; AnsiString Address; int Age; }; // header private: TNamesInfo ni; Then in the forms constructor you init the variables like. ni.Name = ""; ni.Address = ""; ni.Age = 0; How can i do the same procedure with a class? I think in a class, it has its own constructor where you can init the variables without using the forms constructor. Or am i confusing myself by thinking too technical. I have seen in some structs, you will call the struct like TNamesInfo *ni = new TNamesInfo; How come you can do it like that, and you can also use it like: ni.Name = "aname"; I am a bit lost on the usage of all of this, but i don't want to get too ahead of myself from the question i posted. Appreciate the help. Russell Back to top Hans Galema Guest Posted: Mon Dec 22, 2003 5:08 pm    Post subject: Re: Using classes Russell Hinds wrote: Quote: How do you write a class, and initialize the class variables to null when you first call the class variables. Lets use this simple struct for example. Please wrap your lines manually. For what you are asking you don't have to do anything: all class or struct variables will be zero when you create the struct/class with new. Quote: I have seen in some structs, you will call the struct like TNamesInfo *ni = new TNamesInfo; How come you can do it like that, and you can also use it like: ni.Name = "aname"; No you would use it like: ni->Name = "aname"; then. Please tell if you want to initialise to something different as zero or empty. Hans. Back to top JD Guest Posted: Mon Dec 22, 2003 6:18 pm    Post subject: Re: Using classes Hans Galema <dontusethis (AT) dontusethis (DOT) nl> wrote: Quote: Russell Hinds wrote: [...] and you can also use it like: ni.Name = "aname"; No you would use it like: ni->Name = "aname"; then. That would depend how it's being referenced. If by a pointer, then it would be ->. If referencing an instance that was allocated on the stack: TMyStruct MyStruct; MyStruct.Name = "My Name"; ~ JD Back to top Remy Lebeau (TeamB) Guest Posted: Mon Dec 22, 2003 6:49 pm    Post subject: Re: Using classes "Hans Galema" <dontusethis (AT) dontusethis (DOT) nl> wrote Quote: For what you are asking you don't have to do anything: all class or struct variables will be zero when you create the struct/class with new. That is not true at all. The 'new' operator *does not* automatically zero out the allocated memory. You have to do that yourself. You are probably being confused by the fact that *VCL* classes automatically zero out their own memory upon construction. But that is a VCL-specific feature, that is not part of the C++ language itself at all. Gambit Back to top Remy Lebeau (TeamB) Guest Posted: Mon Dec 22, 2003 6:55 pm    Post subject: Re: Using classes "Russell Hinds" <cjhinds (AT) sympatic (DOT) uk> wrote Quote: i am trying to learn about classes, i know its like a struct but a bit different. As far as the compiler is concerned, a 'class' and a 'struct' are exactly the same, except that the members of a 'struct' are 'public' by default if you do not explitically declare 'public', whereas the members of a class are 'private' by default instead. Other than that, anything you can do with a 'class', you can do with a 'struct' and vice versa. Quote: I think in a class, it has its own constructor where you can init the variables without using the forms constructor. Correct. And a 'struct' can have its own constructor as well. struct TNamesInfo { AnsiString Name; AnsiString Address; int Age; TNamesInfo() { Age = 0; } }; In the example above, AnsiString has its own constructor that initializes its memory, so there is no need to set the strings to "" in your own code, since it is already being done for you. You can alternatively use the constructor's initialization list to initialize the 'int' member instead of using an assginment statement inside the constructor body: struct TNamesInfo { AnsiString Name; AnsiString Address; int Age; TNamesInfo() : Age(0) { } }; Quote: I have seen in some structs, you will call the struct like TNamesInfo *ni = new TNamesInfo; How come you can do it like that, and you can also use it like: ni.Name = "aname"; You are invoking the default constructor in both cases. If you do not explitically declare your own constructor, the compiler will create a default one for you. A default constructor is simply one without any parameters. In the 'new' statement above, you are creating the instance on the heap, invoking the default constructor. If you had declared the instance via the statement "TNamesInfo ni;" instead, the same constructor will be invoked, but it will create the instance on the stack instead of the heap. Gambit Back to top JD Guest Posted: Mon Dec 22, 2003 7:17 pm    Post subject: Re: Using classes Hans Galema <dontusethis (AT) dontusethis (DOT) nl> wrote: Quote: No. In the context you can see that ni is declared as a pointer. We're both wrong - and right. One of his questions was exactly this point - that he didn't understand when to use '->' v/s '.'. His code showed allocation on both the heap AND the stack. Russell, if you allocate on the stack: TNamesInfo ni; then you would use ni.Name = "Some Name"; If you allocate from the heap: TNamesInfo *ni = new TNamesInfo; then you would use ni->Name = "Some Name"; You could also (I don't know why you would but you can): TNamesInfo ni; TNamesInfo *pni = (TNamesInfo*)∋ and then these two lines are the same: ni.Name = "Some Name"; pni->Name = "Some Name"; ~ JD Back to top Hans Galema Guest Posted: Mon Dec 22, 2003 8:05 pm    Post subject: Re: Using classes JD wrote: Quote: [...] and you can also use it like: ni.Name = "aname"; No you would use it like: ni->Name = "aname"; then. That would depend how it's being referenced. No. In the context you can see that ni is declared as a pointer. Hans. Back to top Hans Galema Guest Posted: Mon Dec 22, 2003 8:08 pm    Post subject: Re: Using classes Remy Lebeau (TeamB) wrote: Quote: For what you are asking you don't have to do anything: all class or struct variables will be zero when you create the struct/class with new. That is not true at all. The 'new' operator *does not* automatically zero out the allocated memory. You have to do that yourself. You are probably being confused by the fact that *VCL* classes automatically zero out their own memory upon construction. But that is a VCL-specific feature, that is not part of the C++ language itself at all. Aha. But it could also be a compiler feature. Then until now i now I'm lucky seeing structures zeroed with new. Hans.. Back to top Russell Hinds Guest Posted: Mon Dec 22, 2003 9:08 pm    Post subject: Re: Using classes Quote: Correct. And a 'struct' can have its own constructor as well. struct TNamesInfo { AnsiString Name; AnsiString Address; int Age; TTreeNode* Node; TNamesInfo() { Age = 0; Node = NULL; } }; But if i do my struct like that, do i have to use the 'new' with it in order to initialize the constructor? Because i would much rather just use the TNamesInfo ni; instead of having to create and delete the struct. I just tried it, and i actually dont have to use it using the new statement. That is cool, i had not seen a struct like that before. I tried it with a variable of TTreeNode and it actually made it null, where before i had to null it out for myself. Thanks everyone for explaining this to me. I have one more small question if you don't mind. Quote: the heap, invoking the default constructor. If you had declared the instance via the statement "TNamesInfo ni;" instead, the same constructor will be invoked, but it will create the instance on the stack instead of the heap. What is the difference with working on the heap, or from the stack? I am totally lost on that one. Back to top Hans Galema Guest Posted: Mon Dec 22, 2003 10:08 pm    Post subject: Re: Using classes JD wrote: Quote: Hans Galema <dontusethis (AT) dontusethis (DOT) nl> wrote: No. In the context you can see that ni is declared as a pointer. We're both wrong - and right. No, there was nothing wrong in my statement. Quote: TNamesInfo ni; then you would use ni.Name = "Some Name"; If you allocate from the heap: TNamesInfo *ni = new TNamesInfo; then you would use ni->Name = "Some Name"; Correct. Hans. Back to top Duane Hebert Guest Posted: Mon Dec 22, 2003 10:14 pm    Post subject: Re: Using classes Quote: What is the difference with working on the heap, or from the stack? I am totally lost on that one. When creating an instance on the heap, it's necessary to delete the pointer. When creating it on the stack, it's gone when it goes out of scope. Back to top Rodolfo Frino - Macrosoft Guest Posted: Tue Dec 23, 2003 5:14 am    Post subject: Re: Using classes Ther is another difference between struct and classes that you haven't mentioned: The default access specifier for a base class of derived struct is public while for class is private. Rodolfo "Remy Lebeau (TeamB)" <gambit47.no.spam (AT) no (DOT) spam.yahoo.com> wrote Quote: "Russell Hinds" <cjhinds (AT) sympatic (DOT) uk> wrote in message news:3fe717f0$1 (AT) newsgroups (DOT) borland.com... i am trying to learn about classes, i know its like a struct but a bit different. As far as the compiler is concerned, a 'class' and a 'struct' are exactly the same, except that the members of a 'struct' are 'public' by default if you do not explitically declare 'public', whereas the members of a class are 'private' by default instead. Other than that, anything you can do with a 'class', you can do with a 'struct' and vice versa. I think in a class, it has its own constructor where you can init the variables without using the forms constructor. Correct. And a 'struct' can have its own constructor as well. struct TNamesInfo { AnsiString Name; AnsiString Address; int Age; TNamesInfo() { Age = 0; } }; In the example above, AnsiString has its own constructor that initializes its memory, so there is no need to set the strings to "" in your own code, since it is already being done for you. You can alternatively use the constructor's initialization list to initialize the 'int' member instead of using an assginment statement inside the constructor body: struct TNamesInfo { AnsiString Name; AnsiString Address; int Age; TNamesInfo() : Age(0) { } }; I have seen in some structs, you will call the struct like TNamesInfo *ni = new TNamesInfo; How come you can do it like that, and you can also use it like: ni.Name = "aname"; You are invoking the default constructor in both cases. If you do not explitically declare your own constructor, the compiler will create a default one for you. A default constructor is simply one without any parameters. In the 'new' statement above, you are creating the instance on the heap, invoking the default constructor. If you had declared the instance via the statement "TNamesInfo ni;" instead, the same constructor will be invoked, but it will create the instance on the stack instead of the heap. Gambit Back to top Hans Galema Guest Posted: Tue Dec 23, 2003 9:20 am    Post subject: Re: Using classes Hans Galema wrote: Quote: Remy Lebeau (TeamB) wrote: That is not true at all. ... until now i now I'm lucky seeing structures zeroed with new. That was because after new I use initialisers like ZeroMemory(). Sorry for the wrong info. Hans. Back to top Remy Lebeau (TeamB) Guest Posted: Tue Dec 23, 2003 7:30 pm    Post subject: Re: Using classes "Hans Galema" <dontusethis (AT) dontusethis (DOT) nl> wrote Quote: Aha. But it could also be a compiler feature. Perhaps in theory, but in reality it is not. Quote: Then until now i now I'm lucky seeing structures zeroed with new. Correct. You should never have assumed that it would happen automatically. Gambit Back to top Remy Lebeau (TeamB) Guest Posted: Tue Dec 23, 2003 7:35 pm    Post subject: Re: Using classes "Russell Hinds" <cjhinds (AT) sympatic (DOT) uk> wrote Quote: But if i do my struct like that, do i have to use the 'new' with it in order to initialize the constructor? No. Just like a 'class' can invoke any constructor directly when allocating an instance on the stack, the same applies to a 'struct' as well. For example: struct TNamesInfo { AnsiString Name; AnsiString Address; int Age; TTreeNode* Node; TNamesInfo(int Age) { this->Age = Age; Node = NULL; } }; // on the heap TNamesInfo *info = new TNamesInfo(10); // on the stack TNamesInfo info(10); 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 Usage) 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.