BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Virtual Templatized Method Goto page 1, 2  Next          BorlandTalk.com Forum Index -> C++ Builder (Language C++) View previous topic :: View next topic   Author Message tinyabs Guest Posted: Sun Aug 28, 2005 3:33 pm    Post subject: Virtual Templatized Method Please look at the simple code below. template<class T> class Handle { protected: // called by destructor virtual void Close(){...} virtual ~Handle() { Close(); } }; class Monitor : public Handle<LPCRITICAL_SECTION> { protected: // by right should be called when Monitor object destroy void Close() {...} }; The test code is this. { Monitor monitor(new CRITICAL_SECTION); } When monitor object exit the scope, Handle<T>::Close() is called instead of Monitor::Close(). How do I let Monitor::Close() be called instead? Back to top Remy Lebeau (TeamB) Guest Posted: Sun Aug 28, 2005 7:33 pm    Post subject: Re: Virtual Templatized Method "tinyabs" <nospam (AT) me (DOT) com> wrote Quote: virtual void Close(){...} virtual ~Handle() { Close(); } That will not work. A base class destructor cannot call a virtual method that is overridden in a descenant class. By the time the base class destructor is called, the descendant class no longer exists an cannot be accessed. Your descendant class must provide its own destructor that calls Close() directly. Quote: class Monitor : public Handle<LPCRITICAL_SECTION { protected: // by right should be called when Monitor object destroy void Close() {...} No, it should not. Not with the code you have shown. Quote: When monitor object exit the scope, Handle of Monitor::Close(). Correct. That is how virtual destructors work. Quote: How do I let Monitor::Close() be called instead? You cannot. Not from Handle<T> anyway. The only way is for Monitor to declare its own destructor, ie: template< class T > class Handle { protected: virtual void Close() = 0; public: virtual ~Handle() {} }; class Monitor : public Handle<LPCRITICAL_SECTION> { protected: virtual void Close() {...} public: virtual ~Monitor() { Close(); } }; Gambit Back to top tinyabs Guest Posted: Sun Aug 28, 2005 8:22 pm    Post subject: Re: Virtual Templatized Method Thanks. I understood now. Back to top Alan Bellingham Guest Posted: Sun Aug 28, 2005 10:32 pm    Post subject: Re: Virtual Templatized Method "tinyabs" <nospam (AT) me (DOT) com> wrote: Quote: Please look at the simple code below. template<class T class Handle { protected: // called by destructor virtual void Close(){...} virtual ~Handle() { Close(); } }; class Monitor : public Handle { protected: // by right should be called when Monitor object destroy void Close() {...} }; Ah, grasshopper. You have missed the essence of the C++ object model. Base objects are constructed before derived objects, and destroyed after the derived objects are destroyed. Within a destructor, the type of the object being destroyed is _that of the destructor_, not of whatever the final object had been. Hence, Handle Quote: The test code is this. { Monitor monitor(new CRITICAL_SECTION); } When monitor object exit the scope, Handle<T>::Close() is called instead of Monitor::Close(). How do I let Monitor::Close() be called instead? You call the Close() method within ~Monitor(). BTW - within a constructor, the type of object being constructed is also that of the constructor. This is why you may get a pure virtual function call happening. It's almost always down to a constructor or destructor initialing the call. Alan Bellingham -- Me <url:mailto:alanb (AT) episys (DOT) com> <url:http://www.doughnut.demon.co.uk/> ACCU - C, C++ and Java programming <url:http://accu.org/> The 2004 Discworld Convention <url:http://dwcon.org/> Back to top tinyabs Guest Posted: Mon Aug 29, 2005 1:45 am    Post subject: Re: Virtual Templatized Method "Alan Bellingham" <alanb (AT) episys (DOT) com> wrote Quote: "tinyabs" <nospam (AT) me (DOT) com> wrote: You have missed the essence of the C++ object model. C++ was one of the best experiences I had. I'm spending lots of time to learn. Quote: Base objects are constructed before derived objects, and destroyed after the derived objects are destroyed. What's the difference between a virtual and a non-virtual destructor? Whether virtual or not, it seems to have same calling sequence. Quote: Within a destructor, the type of the object being destroyed is _that of the destructor_, not of whatever the final object had been. Hence, Handle<T>::Close() will be called. if I convert it to Handle<T> *, will the derived class virtual method be called? I had <Class> * objects and derived class's virtual method are called this way. Quote: When monitor object exit the scope, Handle<T>::Close() is called instead of Monitor::Close(). How do I let Monitor::Close() be called instead? You call the Close() method within ~Monitor(). BTW - within a constructor, the type of object being constructed is also that of the constructor. Are you saying a derived class is a object inside the base class, technically? And they are classes of their own, semantically in compartments. Back to top Alisdair Meredith[TeamB] Guest Posted: Mon Aug 29, 2005 4:54 am    Post subject: Re: Virtual Templatized Method tinyabs wrote: Quote: What's the difference between a virtual and a non-virtual destructor? Whether virtual or not, it seems to have same calling sequence. That's a good queston <g> The difference is seen when you call delete on a pointer to an object. Try the following simple example: #include <iostream> #include <ostream> struct base { base() { std::cout << "constructing base" << std::endl; Test(); } virtual ~base() { Test(); std::cout << "destroying base" << std::endl; } virtual void Test() { std::cout << "testing base" << std::endl; } }; struct derived : base { derived() : base() { } virtual ~derived() { std::cout << "destroying derived" << std::endl; } virtual void Test() { std::cout << "testing derived" << std::endl; } }; int main() { base * pBase = new derived(); std::cout << std::endl; pBase->Test(); std::cout << std::endl; delete pBase; } When you run this code, you will see that the call to Test from outside the constructor/destructor is dispatched 'dynamically' calling the derived version. However, the calls to Test from the constructor/destructor are dispatched 'statically' and call the version for the class being constructed/destroyed. Likewise, although we have a pointer to a base object, when we call delete it is the derived constructor that is called - because the destructor is virtual. Try the same example but remove the virtual keyword from the base constructor, and you will see the derived destructor is not called. In this example the effect is not too bad, but imagine if derived introduced members or state of its own, such as holding onto a database transaction on a remote server. These resources would never be released. AlisdairM(TeamB) Back to top tinyabs Guest Posted: Mon Aug 29, 2005 7:20 am    Post subject: Re: Virtual Templatized Method Quote: When you run this code, you will see that the call to Test from outside the constructor/destructor is dispatched 'dynamically' calling the derived version. However, the calls to Test from the constructor/destructor are dispatched 'statically' and call the version for the class being constructed/destroyed. I know there's a VMT in each pointer and derived virtual methods are invoked instead of base classes'. But does a non-pointer(what do u call it anyway?) uses VMT or not? Do I really have to use pointer to get derived method to call the way I wanted? Back to top Hendrik Schober Guest Posted: Mon Aug 29, 2005 7:55 am    Post subject: Re: Virtual Templatized Method tinyabs <nospam (AT) me (DOT) com> wrote: Quote: When you run this code, you will see that the call to Test from outside the constructor/destructor is dispatched 'dynamically' calling the derived version. However, the calls to Test from the constructor/destructor are dispatched 'statically' and call the version for the class being constructed/destroyed. I know there's a VMT in each pointer and derived virtual methods are invoked instead of base classes'. But does a non-pointer(what do u call it anyway?) uses VMT or not? Do I really have to use pointer to get derived method to call the way I wanted? What you mean is whether functions are called polymorphicaly (i.e. which function to call is figured out at run-time) or directly. The former is only in place when, at run-time, it is unknown which type the object has. If you have Derived d; d.f(); the type is well-known. If, OTOH, you have a pointer or a reference to a base class, the type isn't know -- the might be an instance of some derived class behind the ptr/ref: Base& rb = d; Base* pb = &d; rb.f(); pb->f(); In this case, if 'Base::f()' is virtual, it's called through the VMT. HTH, Schobi -- [email]SpamTrap (AT) gmx (DOT) de[/email] is never read I'm Schobi at suespammers dot org "Coming back to where you started is not the same as never leaving" Terry Pratchett Back to top Hendrik Schober Guest Posted: Mon Aug 29, 2005 8:02 am    Post subject: Re: Virtual Templatized Method tinyabs <nospam (AT) me (DOT) com> wrote: Quote: [...] BTW - within a constructor, the type of object being constructed is also that of the constructor. Are you saying a derived class is a object inside the base class, technically? And they are classes of their own, semantically in compartments. No. He said that the dynamic type of object, during construction/destruction of the base class, is the type of the base class, because at that time, the object of the derived class is either not yet constructed (derived class ctors are called after base class ctors) or no longer existing (derived class dtors are called before base class dtors). You don't need to memorize this, BTW, as it is very logically: When inside of a derived class' ctor, you expect to have the base class parts of the object fully constructed. For example: class Base { // ... X* p; }; class Derived { // ... }; When inside 'Derived::Derived()', you expect 'Base::p' to be initialized by 'Base::Base()'. So 'Base::Base()' has to be called before 'Derived::Derived()'. Once you figured out this, just remember that dtors are called in the opposite order. Schobi -- [email]SpamTrap (AT) gmx (DOT) de[/email] is never read I'm Schobi at suespammers dot org "Coming back to where you started is not the same as never leaving" Terry Pratchett Back to top Alan Bellingham Guest Posted: Mon Aug 29, 2005 9:49 am    Post subject: Re: Virtual Templatized Method "tinyabs" <nospam (AT) me (DOT) com> wrote: Quote: Base objects are constructed before derived objects, and destroyed after the derived objects are destroyed. What's the difference between a virtual and a non-virtual destructor? Whether virtual or not, it seems to have same calling sequence. The difference between a virtual and a non-virtual destructor is that, given a pointer to base, "delete base;" will call the derived destructor if and only if it is a virtual destructor. Quote: Within a destructor, the type of the object being destroyed is _that of the destructor_, not of whatever the final object had been. Hence, Handle<T>::Close() will be called. if I convert it to Handle<T> *, will the derived class virtual method be called? I had <Class> * objects and derived class's virtual method are called this way. The derived class destructor will be called. But it was being called any, since it is virtual. However, it is in the base class destructor that the call to Close() is made, and that is after the derived class no longer exists. Quote: Are you saying a derived class is a object inside the base class, technically? And they are classes of their own, semantically in compartments. The other way around. Technically speaking, the base class is a sub-object of the derived object. During construction of an object, all subobjects are constructed first. This means all base classes, and all member objects. Alan Bellingham -- Me <url:mailto:alanb (AT) episys (DOT) com> <url:http://www.doughnut.demon.co.uk/> ACCU - C, C++ and Java programming <url:http://accu.org/> The 2004 Discworld Convention <url:http://dwcon.org/> Back to top tinyabs Guest Posted: Mon Aug 29, 2005 12:44 pm    Post subject: Re: Virtual Templatized Method Quote: No. He said that the dynamic type of object, during construction/destruction of the base class, is the type of the base class, because at that time, the object of the derived class is either not yet constructed (derived class ctors are called after base class ctors) or no longer existing (derived class dtors are called before base class dtors). You don't need to memorize this, BTW, as it is very logically: When inside of a derived class' ctor, you expect to have the base class parts of the object fully constructed. So obviously, I can't call a derived method in base method or access a private scope derived variable from base class. They are actually in compartments. Thanks guys. Back to top Hendrik Schober Guest Posted: Mon Aug 29, 2005 12:51 pm    Post subject: Re: Virtual Templatized Method tinyabs <nospam (AT) me (DOT) com> wrote: Quote: No. He said that the dynamic type of object, during construction/destruction of the base class, is the type of the base class, because at that time, the object of the derived class is either not yet constructed (derived class ctors are called after base class ctors) or no longer existing (derived class dtors are called before base class dtors). You don't need to memorize this, BTW, as it is very logically: When inside of a derived class' ctor, you expect to have the base class parts of the object fully constructed. So obviously, I can't call a derived method in base method or access a private scope derived variable from base class. [...] Yes, you can. You only cannot during the construction/destruction of the base object. Quote: Thanks guys. Schobi -- [email]SpamTrap (AT) gmx (DOT) de[/email] is never read I'm Schobi at suespammers dot org "Coming back to where you started is not the same as never leaving" Terry Pratchett Back to top Chris Uzdavinis (TeamB) Guest Posted: Mon Aug 29, 2005 1:49 pm    Post subject: Re: Virtual Templatized Method "tinyabs" <nospam (AT) me (DOT) com> writes: Quote: I know there's a VMT in each pointer Not quite right. There is a VMT in each CLASS that has a virtual function. Each object of such a class has an invisible pointer to its VMT. When you call a function on a pointer whose type is polymorphic, the runtime will dereference your pointer to find its object, pull out the pointer to the VMT for that class, dereference that pointer to see the table, and fetch the function pointer stored in the proper offset. Quote: and derived virtual methods are invoked instead of base classes'. Whatever function is in the class's VTBL (VMT) is used. Normally a derived class gets an exact copy from its base class, unless it overrides a function. Then the overridden function replaces the base pointer in the table. For functions not overridden, the base version is called, and for new virtual functions, the derived class's VTBL is expanded. Quote: But does a non-pointer(what do u call it anyway?) uses VMT or not? A reference to an object uses the virtual lookup mechanism, but calling directly on an object does not, since the object already is the most derived type and thus there is no need to lookup the virtual function in the VTBL. Quote: Do I really have to use pointer to get derived method to call the way I wanted? You can use a pointer or reference. But, be sure that the pointer points to a valid object! If you're in the destructor of a base class, don't downcast it to call a derived function, because the derived part has already been destroyed and the program will have undefined behavior. -- Chris (TeamB); Back to top tinyabs Guest Posted: Tue Aug 30, 2005 6:40 am    Post subject: Re: Virtual Templatized Method ----------------------------- library code ------------------------------------ template<class T> class Handle { protected: struct HandleRef { T Handle; int RefCount; }; HandleRef *FRefObj; void AddRef() { if (FRefObj != NULL) ++ FRefObj->RefCount; } void Release() { if (FRefObj != NULL) { -- FRefObj->RefCount; if (FRefObj->RefCount == 0) { Close(); FRefObj->Handle = NULL; delete FRefObj; FRefObj = NULL; } } } virtual void Close() = 0; public: explicit Handle(const HANDLE handle) { CopyNew(handle); } Handle() { FRefObj = NULL; } Handle(const Handle &handle) { Copy(handle); } virtual ~Handle() { Release(); } void Copy(const Handle &handle) { Release(); FRefObj = handle.FRefObj; AddRef(); } void CopyNew(T &handle) { FRefObj = new HandleRef; FRefObj->Handle = handle; FRefObj->RefCount = 1; } void Reset(T handle = NULL) { Release(); if (handle != NULL) { CopyNew(handle); } else FRefObj = NULL; } T Get() const { return FRefObj->Handle; } Handle &operator=(Handle &handle) { Copy(handle); } operator T() const { return FRefObj->Handle; } private: }; class Monitor : public Handle<LPCRITICAL_SECTION> { public: Monitor(); Monitor(const Monitor &monitor); ~Monitor(); void Enter(); bool TryEnter(); void Leave(); Monitor &operator=(Monitor &monitor); protected: void Close(); private: }; -------------------- Demo Code------------------------ { Monitor monitor; } Any alternative to get Monitor::Close() called? "Chris Uzdavinis (TeamB)" <chris (AT) uzdavinis (DOT) com> wrote Quote: "tinyabs" <nospam (AT) me (DOT) com> writes: I know there's a VMT in each pointer Not quite right. There is a VMT in each CLASS that has a virtual function. Each object of such a class has an invisible pointer to its VMT. When you call a function on a pointer whose type is polymorphic, the runtime will dereference your pointer to find its object, pull out the pointer to the VMT for that class, dereference that pointer to see the table, and fetch the function pointer stored in the proper offset. and derived virtual methods are invoked instead of base classes'. Whatever function is in the class's VTBL (VMT) is used. Normally a derived class gets an exact copy from its base class, unless it overrides a function. Then the overridden function replaces the base pointer in the table. For functions not overridden, the base version is called, and for new virtual functions, the derived class's VTBL is expanded. But does a non-pointer(what do u call it anyway?) uses VMT or not? A reference to an object uses the virtual lookup mechanism, but calling directly on an object does not, since the object already is the most derived type and thus there is no need to lookup the virtual function in the VTBL. Do I really have to use pointer to get derived method to call the way I wanted? You can use a pointer or reference. But, be sure that the pointer points to a valid object! If you're in the destructor of a base class, don't downcast it to call a derived function, because the derived part has already been destroyed and the program will have undefined behavior. -- Chris (TeamB); Back to top Chris Uzdavinis (TeamB) Guest Posted: Tue Aug 30, 2005 1:10 pm    Post subject: Re: Virtual Templatized Method "tinyabs" <nospam (AT) me (DOT) com> writes: Quote: Any alternative to get Monitor::Close() called? The way to accomplish that is to call it from Monitor::~Monitor(); -- Chris (TeamB); 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 (Language C++) 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.