C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Why it's a pointer not just a class          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Michael Guest Posted: Thu Aug 24, 2006 7:24 am    Post subject: Why it's a pointer not just a class Hi, typedef std::vector<Vehicle*> VehicleList; Vehicle is a UDT, or a class. Why is Vehicle* not just Vehicle in < >. Which one is better and why? If you could explain it by laying out some codes, it would be highly appreciated! Thanks in advance, Michael Back to top Daniel T. Guest Posted: Thu Aug 24, 2006 8:08 am    Post subject: Re: Why it's a pointer not just a class "Michael" <michaelnx (AT) gmail (DOT) com> wrote: Quote: Hi, typedef std::vector<Vehicle*> VehicleList; Vehicle is a UDT, or a class. Why is Vehicle* not just Vehicle in < >. Which one is better and why? If you could explain it by laying out some codes, it would be highly appreciated! class Vehicle { }; class Car : public Vehicle { }; vector<Vehicle> objVec; objVec.push_back( Car() ); // bad, slices all Car behavior off of the object vector<Vehicle*> ptrVec; ptrVec.push_back( new Car() ); // good, all Car behavior is maintained The problem with the second construct is that the vector doesn't know when to delete the objects it contains. Some other object must track that. This is often done with some sort of smart pointer. vector< shared_ptr< Vehicle > > smartVec; Back to top Salt_Peter Guest Posted: Thu Aug 24, 2006 9:04 am    Post subject: Re: Why it's a pointer not just a class Michael wrote: Quote: Hi, typedef std::vector<Vehicle*> VehicleList; Vehicle is a UDT, or a class. Why is Vehicle* not just Vehicle in < >. Which one is better and why? If you could explain it by laying out some codes, it would be highly appreciated! Thanks in advance, Michael Because a Vehicle is abstract. Your goal here is to create a container that holds derivatives of the Vehicle class. A Car is a type of vehicle. A Truck is a type of Vehicle. A Bus is a type of Vehicle. Why have to create a new container for each new type when a container of Vehicles can store any type that is related to Vehicles, including a mix of Vehicles? Polymorphicly, you are creating a container of pointers to Vehicle derivatives. That pointer to each element then allows the appropriate behaviour to happen: brake(), reverse(), accelerate(). For example, a car will not reverse() like a truck - a truck needs to sound a warning buzzer first. The pointer essentially lets the vtable do its "magic". As far as code is concerned: #include <iostream> #include <ostream> #include <vector> class Vehicle { public: Vehicle() { } virtual ~Vehicle() { } // virtual virtual void reverse() = 0; // pure virtual -- see below }; class Car : public Vehicle { public: Car() { } ~Car() { std::cout << "~Car()" << std::endl; } void reverse() { std::cout << "reversing" << std::endl; } }; class Truck : public Vehicle { public: Truck() { } ~Truck() { std::cout << "~Truck()" << std::endl; } void reverse() { std::cout << "buzzer, reversing" << std::endl; } }; int main() { std::vector< Vehicle* > vehicles; // vehicles.push_back( new Vehicle ); error - Vehicle is abstract vehicles.push_back( new Car ); vehicles.push_back( new Truck ); vehicles[ 0 ]->reverse(); vehicles[ 1 ]->reverse(); for ( int i = 0; i < vehicles.size(); ++i ) // or size_t { delete vehicles[ i ]; } return 0; } /* reversing buzzer, reversing ~Car() ~Truck() */ The destructors are virtual too, they have to be for this to work. The reason that reverse() was declared pure virtual in Vehicle is to force the client programmer to provide a reverse() member function in order to use this system. Note that i can write a SportsCar class which derives from Car and still use it in the vehicles container. A SportsCar is_a Car which is_a Vehicle, hence a SportsCar is_a Vehicle. If you don't yet see the benefits of such a system, consider encapsulating the std::vector in a container class: all of a sudden, you'ld have a container that can hold any Vehicle - including those derivatives that have not been created yet (ie: SuperSportsCar or TenTonTruck). Without having to modify the container in any way. Thats a very powerful concept - your code becomes extensible without having to modify the original classes. Back to top Heinz Ozwirk Guest Posted: Thu Aug 24, 2006 9:10 am    Post subject: Re: Why it's a pointer not just a class "Michael" <michaelnx (AT) gmail (DOT) com> schrieb im Newsbeitrag news:1156386253.269170.250710 (AT) i42g2000cwa (DOT) googlegroups.com... Quote: Hi, typedef std::vector<Vehicle*> VehicleList; Vehicle is a UDT, or a class. Why is Vehicle* not just Vehicle in < >. Look how Vehicle is defined and how it is used. Quote: Which one is better and why? Which one is better - a hammer or a tooth brush - and why? None is better than the other. It depends on what you want to do. Heinz Back to top Roland Pibinger Guest Posted: Tue Aug 29, 2006 9:10 am    Post subject: Re: Why it's a pointer not just a class On Tue, 29 Aug 2006 00:58:37 +0200, "Bo Persson" <bop (AT) gmb (DOT) dk> wrote: Quote: You shouldn't store objects that are expensive to copy in a standard container. It's not mainly a question whether the container elements are expensive to copy. The important distinction is between 'values' and 'objects' (objects in the OO sense). Objects are usually non-copyable but may be accessed through multiple references (pointers in C++). STL suppports only values. That causes the problems. Best wishes, Roland Pibinger Back to top I V Guest Posted: Wed Aug 30, 2006 9:10 am    Post subject: Re: Why it's a pointer not just a class On Tue, 29 Aug 2006 03:30:08 -0700, Earl Purple wrote: Quote: and then there is, of course, boost: [snip] Not to mention boost::indirect_iterator, http://boost.org/libs/iterator/doc/indirect_iterator.html I'm not familiar with the Rogue Wave library, but a quick glance at the documentation suggests that combining the standard containers with boost::indirect_iterator accomplishes most, if not all, of what the RW pointer containers do (from what I can tell, the RW classes don't deal with ownership issues, which is probably the hardest part of a pointer container to figure out). Back to top Roland Pibinger Guest Posted: Wed Aug 30, 2006 9:11 am    Post subject: Re: Why it's a pointer not just a class On Tue, 29 Aug 2006 22:07:04 -0700, I V <wrongbad (AT) gmail (DOT) com> wrote: Quote: On Tue, 29 Aug 2006 03:30:08 -0700, Earl Purple wrote: Not to mention boost::indirect_iterator, http://boost.org/libs/iterator/doc/indirect_iterator.html I'm not familiar with the Rogue Wave library, but a quick glance at the documentation suggests that combining the standard containers with boost::indirect_iterator accomplishes most, if not all, of what the RW pointer containers do Nope. When you use eg. std::sort with an boost::indirect_iterator it swaps pointed-to objects not pointers. Containers, iterators and algorithms need to become pointer aware for a real solution. Moreover, the STL templates are notorious for their compiler error messages. An iterator adapter adds another level of obfuscation to the program. Quote: (from what I can tell, the RW classes don't deal with ownership issues, which is probably the hardest part of a pointer container to figure out). Why should a container for pointers be usable only for objects allocated with new? Why is such a container supposed to delete contained objects? A basic library container should not intermingle container functionality and ownership. It should be usable for all pointers (to heap-allocatd, to stack and to global objects). IMO, it's a design flaw to write containers that insert pointers and require that these pointers _must_ point to heap-allocated objects (which are later deleted by the container). Of course, one can implement ownership semantics on top of a basic pointer container but to build ownership (and transfer of ownership to the container) into the container is (put mildly) a highly questionable design decision. Best wishes, Roland Pibinger Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ language (comp.lang.c++) All times are GMT Page 1 of 1   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  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.