C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  What is the correct way to derive a class in regard to overl          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Guest Posted: Fri May 19, 2006 10:21 am    Post subject: What is the correct way to derive a class in regard to overl Hi, I am trying to derive a new class that will add new functions but no new data members and the base class has overloaded operators (+,-,+=,-=,etc...) returning either (Base &) or (const Base) depending on the operator: class Derived : public Base { }; The problem occurs if I do the following Derived d1,d2; Derived d3 = d1+d2; what should be the better approach? Just add Derived(const Base&) and Derived &operator=(const Base&) or should it be better to redeclare all the operators (there is a lot of them) in the derived class and perform nothing except calling the base class version and return Derived reference or object. Greetings, Olivier Langlois http://www3.sympatico.ca/olanglois [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kwikius Guest Posted: Sat May 20, 2006 9:21 pm    Post subject: Re: What is the correct way to derive a class in regard to o olanglois (AT) sympatico (DOT) ca wrote: Quote: Hi, I am trying to derive a new class that will add new functions but no new data members and the base class has overloaded operators (+,-,+=,-=,etc...) returning either (Base &) or (const Base) depending on the operator: class Derived : public Base { }; The problem occurs if I do the following Derived d1,d2; Derived d3 = d1+d2; what should be the better approach? Just add Derived(const Base&) and Derived &operator=(const Base&) or should it be better to redeclare all the operators (there is a lot of them) in the derived class and perform nothing except calling the base class version and return Derived reference or object. IMO Always overload the operators for the derived versions if you can, otherwise each operation is effectively losing type information and possibly the deriveds extra data. In fact dont bother overloading them for the base class so you get an error if you havent. Of course thats more work... so you could try looking at ; http://www.boost.org/libs/utility/operators.htm which is meant to reduce the work required to implement common operators. (No I havent used it myself) regards Andy Little [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Sat May 20, 2006 9:21 pm    Post subject: Re: What is the correct way to derive a class in regard to o In article <1147983459.860594.62750 (AT) j55g2000cwa (DOT) googlegroups.com>, olanglois (AT) sympatico (DOT) ca writes Quote: Hi, I am trying to derive a new class that will add new functions but no new data members and the base class has overloaded operators (+,-,+=,-=,etc...) returning either (Base &) or (const Base) depending on the operator: class Derived : public Base { }; The problem occurs if I do the following Derived d1,d2; Derived d3 = d1+d2; what should be the better approach? You almost certainly do not want to do this by public derivation as I very much doubt that Base was designed as a base class (i.e. I expect it to lack a virtual dtor) Why do you want a class that is identical to Base other than for some added functionality? Why not just add some free functions? Your derived class will have no special access to the base class so its member functions cannot do anything that free functions could not do. Before I give you any further guidance I need to know why you want to do what you are doing. -- Francis Glassborow ACCU Author of 'You Can Do It!' see http://www.spellen.org/youcandoit For project ideas and contributions: http://www.spellen.org/youcandoit/projects [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Bob Hairgrove Guest Posted: Sat May 20, 2006 10:21 pm    Post subject: Re: What is the correct way to derive a class in regard to o On 19 May 2006 06:13:44 -0400, olanglois (AT) sympatico (DOT) ca wrote: Quote: Hi, I am trying to derive a new class that will add new functions but no new data members and the base class has overloaded operators (+,-,+=,-=,etc...) returning either (Base &) or (const Base) depending on the operator: class Derived : public Base { }; The problem occurs if I do the following Derived d1,d2; Derived d3 = d1+d2; what should be the better approach? Just add Derived(const Base&) and Derived &operator=(const Base&) or should it be better to redeclare all the operators (there is a lot of them) in the derived class and perform nothing except calling the base class version and return Derived reference or object. Sounds like a bit of a design problem to me. What control do you have over the design of your Base class? Any operators or other functions which return a Base object will be a potential problem WRT slicing. -- Bob Hairgrove NoSpamPlease (AT) Home (DOT) com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top James Kanze Guest Posted: Mon May 22, 2006 2:21 pm    Post subject: Re: What is the correct way to derive a class in regard to olanglois (AT) sympatico (DOT) ca wrote: Quote: I am trying to derive a new class that will add new functions but no new data members and the base class has overloaded operators (+,-,+=,-=,etc...) returning either (Base &) or (const Base) depending on the operator: class Derived : public Base { }; I'm not sure what you're doing is a good idea. Operator overloading largely depends on value semantics, where as derivation practically requires reference semantics to work correctly. Quote: The problem occurs if I do the following Derived d1,d2; Derived d3 = d1+d2; what should be the better approach? Just add Derived(const Base&) and Derived &operator=(const Base&) or should it be better to redeclare all the operators (there is a lot of them) in the derived class and perform nothing except calling the base class version and return Derived reference or object. I'm not sure why you are deriving, so it's hard to say, but I suspect that the cleanest approch would use containment, not inheritance, and redefine every operator. It's a lot of copy work, of course, but one way or the other, I don't see any way around it. -- James Kanze kanze.james (AT) neuf (DOT) fr Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top James Kanze Guest Posted: Mon May 22, 2006 2:21 pm    Post subject: Re: What is the correct way to derive a class in regard to Francis Glassborow wrote: Quote: In article <1147983459.860594.62750 (AT) j55g2000cwa (DOT) googlegroups.com>, olanglois (AT) sympatico (DOT) ca writes I am trying to derive a new class that will add new functions but no new data members and the base class has overloaded operators (+,-,+=,-=,etc...) returning either (Base &) or (const Base) depending on the operator: class Derived : public Base { }; The problem occurs if I do the following Derived d1,d2; Derived d3 = d1+d2; what should be the better approach? You almost certainly do not want to do this by public derivation as I very much doubt that Base was designed as a base class (i.e. I expect it to lack a virtual dtor) Why do you want a class that is identical to Base other than for some added functionality? Why not just add some free functions? Your derived class will have no special access to the base class so its member functions cannot do anything that free functions could not do. Good point. I just automatically assumed that one of his goals was to have a new type, which of course supposes a class, but if that's not the case, free functions are the way to go. There is also one very special case I've used from time to time: derivation to provide new constructors. But in such cases, "slicing" is perfectly acceptable -- once the constructor has run, I simply use the object as if it were a base, copying the base part, etc. without worrying that the class was originally constructed as a derived. -- James Kanze kanze.james (AT) neuf (DOT) fr Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Guest Posted: Tue May 23, 2006 11:21 am    Post subject: Re: What is the correct way to derive a class in regard to o Hi, Thanks all for your input. I am going to answer to your questions to keep the ball rolling. I have 100% control over the base class and I'm willing to change anything to come up with the best solution. Here is some background on what I try to achieve. I have class with a default constructor defined like this: Base( int param = DEFAULTVAL); I have reused that class in a new program where I would like to override DEFAULTVALUE to something else when I create arrays of this class so the way I came up to solve this problem is by doing this: templace<int DEFAULT = DEFAULTVAL> class Derived : public Base { Derived( int param = DEFAULT ); }; I did not changed Base to become a template because only object construction needs to be templatized. However by doing so, I introduced a new problem with Base operators. Since my original post, for now, to solve the problem is this: 1- Made Base abstract 2- Keep operators returning references in Base and move operators returning objects to Derived. 3- Added the constructor Derived( const Base &) Derived( const Derived&) (By experience, I found out that I needed both) 4- Added the assignment opertor=(const Base &) (otherwise a temporary Derived object is created...) Thanks for your help! Greetings, Olivier Langlois http://www3.sympatico.ca/olanglois [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Marek Vondrak Guest Posted: Tue May 23, 2006 11:21 am    Post subject: Re: What is the correct way to derive a class in regard to o Quote: You almost certainly do not want to do this by public derivation as I very much doubt that Base was designed as a base class (i.e. I expect it to lack a virtual dtor) Why do you want a class that is identical to Base other than for some added functionality? Why not just add some free functions? Your derived class will have no special access to the base class so its member functions cannot do anything that free functions could not do. Before I give you any further guidance I need to know why you want to do what you are doing. Both Base and Derived are probably meant to be used as value types that are allocated on the stack (hence no need for a virtual destructor). Base class might wrap an integral type to a class and define operations so that it could be used in place of the native integer type and Derived might public-inherit from Base and extend its functionality by adding a support for serialization, etc. The question of reimplementing operations in Derived is pretty reasonable in this context then. As for the original question... I am afraid that all operations have to be reimplemented in Derived as stubs delegating the calls to Base, because Derived's operator+() should really return Derived, for example. -- Marek [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Philippe Amarenco Guest Posted: Tue May 23, 2006 11:21 am    Post subject: Re: What is the correct way to derive a class in regard to olanglois (AT) sympatico (DOT) ca writes: Quote: Hi, I am trying to derive a new class that will add new functions but no new data members and the base class has overloaded operators (+,-,+=,-=,etc...) returning either (Base &) or (const Base) depending on the operator: class Derived : public Base { }; The problem occurs if I do the following Derived d1,d2; Derived d3 = d1+d2; what should be the better approach? Just add Derived(const Base&) and Derived &operator=(const Base&) or should it be better to redeclare all the operators (there is a lot of them) in the derived class and perform nothing except calling the base class version and return Derived reference or object. if you can change the code you of Base and want to create a few different Derived class, you probably want to have your base class be a template parametered by your derived class: template <typename T> class Base { T& operator+(T&); // ... }; class Derived : public Base<Derived> { // ... }; -- Philippe Amarenco, aka Phix epita 2007 - GISTR - LSE - EpX [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Tue May 23, 2006 10:21 pm    Post subject: Re: What is the correct way to derive a class in regard to o In article <e4o1uv$2vrn$1 (AT) ns (DOT) felk.cvut.cz>, Marek Vondrak <none (AT) none (DOT) com> writes Quote: Both Base and Derived are probably meant to be used as value types that are allocated on the stack (hence no need for a virtual destructor). Base class might wrap an integral type to a class and define operations so that it could be used in place of the native integer type and Derived might public-inherit from Base and extend its functionality by adding a support for serialization, etc. The question of reimplementing operations in Derived is pretty reasonable in this context then. In the circles I move in, public inheritance of a value type is considered suspect. Public inheritance represents an 'is-a' relationship with the concept of LSP. If the derived type is intended to be a full value type the risk of slicing (i.e unintentional conversion to the base type) provides an opportunity for subtle bugs. Of course what we really need is strong typedefs such as those being considered for C++0x where issues of convertability are tackled by the form of the typedef. -- Francis Glassborow ACCU Author of 'You Can Do It!' see http://www.spellen.org/youcandoit For project ideas and contributions: http://www.spellen.org/youcandoit/projects [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze Guest Posted: Thu May 25, 2006 2:21 pm    Post subject: Re: What is the correct way to derive a class in regard to olanglois (AT) sympatico (DOT) ca wrote: Quote: I have 100% control over the base class and I'm willing to change anything to come up with the best solution. Here is some background on what I try to achieve. I have class with a default constructor defined like this: Base( int param = DEFAULTVAL); I have reused that class in a new program where I would like to override DEFAULTVALUE to something else That's trivial... Just do it, and don't worry about the slicing. The Base operators will still take a Derived, and you should use Base just about everywhere except when you need to construct. (Definitions like: Base obj = Derived() ; are perfectly legal.) Quote: when I create arrays of this class As long as the arrays are std::vector, no problem. A C style array, on the other hand, might be. Quote: so the way I came up to solve this problem is by doing this: templace<int DEFAULT = DEFAULTVAL class Derived : public Base { Derived( int param = DEFAULT ); }; I did not changed Base to become a template because only object construction needs to be templatized. However by doing so, I introduced a new problem with Base operators. Since my original post, for now, to solve the problem is this: 1- Made Base abstract I'm not sure what this buys you. Quote: 2- Keep operators returning references in Base and move operators returning objects to Derived. 3- Added the constructor Derived( const Base &) Derived( const Derived&) (By experience, I found out that I needed both) The compiler should provide the second, if you don't. Quote: 4- Added the assignment opertor=(const Base &) (otherwise a temporary Derived object is created...) You also need to make the destructor of Base virtual. I'd also add a private operator new[] to Derived -- in no case can you allow the construction of C style arrays of Derived on the heap (since they will fatally end up assigned to a Base*). -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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