C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  pure virtual          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Guest Posted: Sat Sep 27, 2003 5:13 pm    Post subject: pure virtual I don't understand. What's the difference in practice between 2 next lines? When I *must* use first line and when second? ------------- virtual int a(int b) = 0; virtual int a(int b); ------------- thank you Back to top Siana Guest Posted: Sat Sep 27, 2003 5:39 pm    Post subject: Re: pure virtual On Sat, 27 Sep 2003 20:13:31 +0300, "<- Chameleon ->" <cham_gss (AT) hotmail (DOT) NOSPAM.com> wrote: Quote: I don't understand. What's the difference in practice between 2 next lines? When I *must* use first line and when second? ------------- virtual int a(int b) = 0; virtual int a(int b); ------------- thank you The first line declares a function a in some class that has no definition - it is there only to be overridden by derived classes for the purposes of polymorphism. It also makes the class abstract (unable to be instantiated). Derived classes must declare and implement this function or else they too are abstract and un-instatiable. It is useful in defining an interface. The second line declares a function a in some class that must have an implementation for that class. That implementation will be inherited by derived classes but can still be overridden and perform polymorphically. S. Back to top Thomas Wintschel Guest Posted: Sat Sep 27, 2003 6:49 pm    Post subject: Re: pure virtual "<- Chameleon ->" <cham_gss (AT) hotmail (DOT) NOSPAM.com> wrote Quote: I don't understand. What's the difference in practice between 2 next lines? When I *must* use first line and when second? ------------- virtual int a(int b) = 0; virtual int a(int b); ------------- thank you You may want to look up 'pure virtual' The first one would be used when defining a pure virtual base class. This is done in situations where code needs to call the method 'a' for objects of types which derive from the class but where you need the derived class to implement the actual method (derived class *must* implement the method). If we define: class C { Back to top Kevin Goodsell Guest Posted: Sat Sep 27, 2003 7:03 pm    Post subject: Re: pure virtual Siana wrote: Quote: On Sat, 27 Sep 2003 20:13:31 +0300, "<- Chameleon ->" [email]cham_gss (AT) hotmail (DOT) NOSPAM.com[/email]> wrote: I don't understand. What's the difference in practice between 2 next lines? When I *must* use first line and when second? ------------- virtual int a(int b) = 0; virtual int a(int b); ------------- thank you The first line declares a function a in some class that has no definition That is not true in general. A pure virtual function certainly may have a definition. Quote: - it is there only to be overridden by derived classes for the purposes of polymorphism. It also makes the class abstract (unable to be instantiated). Derived classes must declare and implement this function or else they too are abstract and un-instatiable. It is useful in defining an interface. -Kevin -- My email address is valid, but changes periodically. To contact me please use the address from a recent posting. Back to top Ron Natalie Guest Posted: Mon Sep 29, 2003 2:00 pm    Post subject: Re: pure virtual "Siana" <siana_canadaREMOVETHISTOSEND (AT) yahoo (DOT) ca> wrote Quote: The first line declares a function a in some class that has no definition Wrong answer. It is the designation of the function as pure virtual. It does not preclude the function from having a definition. It makes the class it is defined in abstract. An abstract class is one that can not be instantiated. It serves only as a base class for others. Quote: - it is there only to be overridden by derived classes for the purposes of polymorphism. Not quite, it means that it must be overridden in dervied classes (as well as all other pure virtual functions) in order to be concrete (that is, one that can have instances created of it). Quote: The second line declares a function a in some class that must have an implementation for that class. That implementation will be inherited by derived classes but can still be overridden and perform polymorphically. No it just declares one that is not pure. It means that it doesn't make the class abstract. While it is the case that you can omit the implementation of a pure virtual function, that is secondary to it's meaning. Back to top jeffc Guest Posted: Mon Sep 29, 2003 2:37 pm    Post subject: Re: pure virtual "<- Chameleon ->" <cham_gss (AT) hotmail (DOT) NOSPAM.com> wrote Quote: I don't understand. What's the difference in practice between 2 next lines? When I *must* use first line and when second? ------------- virtual int a(int b) = 0; virtual int a(int b); ------------- There is no time when you *must* use the first (pure virtual). It's a design decision on your part. There are 2 reasons you might use it a) you want to force any class that inherits from this class to define function a b) you want that class to be abstract (no one can make an instance of it - you can only make instances of subclasses). This is a rather confusing means of creating an abstract class, but that's the way it is. Back to top jeffc Guest Posted: Mon Sep 29, 2003 2:41 pm    Post subject: Re: pure virtual "Thomas Wintschel" <thomwint (AT) telus (DOT) net> wrote Quote: The first one would be used when defining a pure virtual base class. You mean an abstract class. Back to top Ron Natalie Guest Posted: Mon Sep 29, 2003 3:04 pm    Post subject: Re: pure virtual "jeffc" <nobody (AT) nowhere (DOT) com> wrote Quote: "Thomas Wintschel" <thomwint (AT) telus (DOT) net> wrote in message news:KYkdb.12037$o21.6208 (AT) edtnps84 (DOT) .. The first one would be used when defining a pure virtual base class. You mean an abstract class. Now you're being pedantic Back to top jeffc Guest Posted: Mon Sep 29, 2003 3:35 pm    Post subject: Re: pure virtual "Ron Natalie" <ron (AT) sensor (DOT) com> wrote Quote: "jeffc" <nobody (AT) nowhere (DOT) com> wrote "Thomas Wintschel" <thomwint (AT) telus (DOT) net> wrote in message news:KYkdb.12037$o21.6208 (AT) edtnps84 (DOT) .. The first one would be used when defining a pure virtual base class. You mean an abstract class. Now you're being pedantic I knew somone was going to say that The point to my way of thinking is that "pure virtual base class" is not an OO term, it's not a C++ term, it's just not any term I've ever heard. I did not of course object to "method", nor would I even have objected if he used the term "abstract method" for pure virtual function. I'd be interested in knowing if that's what Thomas really meant to write and where he heard it. Maybe I should have said "You mean an abstract class, right?" Back to top Ron Natalie Guest Posted: Mon Sep 29, 2003 3:58 pm    Post subject: Re: pure virtual "jeffc" <nobody (AT) nowhere (DOT) com> wrote Quote: "Ron Natalie" <ron (AT) sensor (DOT) com> wrote in message news:3f7849ee$0$36943$9a6e19ea (AT) news (DOT) newshosting.com... "jeffc" <nobody (AT) nowhere (DOT) com> wrote in message news:3f784563_2 (AT) news1 (DOT) prserv.net... "Thomas Wintschel" <thomwint (AT) telus (DOT) net> wrote in message news:KYkdb.12037$o21.6208 (AT) edtnps84 (DOT) .. The first one would be used when defining a pure virtual base class. You mean an abstract class. Now you're being pedantic :-) I knew somone was going to say that I was just ribbing ya. 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