C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Compile-time detection of abstract base classes          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message J Lindrud Guest Posted: Tue Jan 20, 2004 11:25 am    Post subject: Compile-time detection of abstract base classes Hi! If I have a type T& and I want to dereference it and create an object on the stack, ie "T t();", I run into problems when T happens to be an abstract base class ("error: can't instantiate abstract base class"). So in order to avoid this error, I'm wondering if there is a way of determining at compile time whether a class is an abstract base class or not? Jarl. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Pavel Vozenilek Guest Posted: Wed Jan 21, 2004 9:04 am    Post subject: Re: Compile-time detection of abstract base classes [email]jlindrud (AT) hotmail (DOT) com[/email] (J Lindrud) wrote in message Quote: If I have a type T& and I want to dereference it and create an object on the stack, ie "T t();", I run into problems when T happens to be an abstract base class ("error: can't instantiate abstract base class"). So in order to avoid this error, I'm wondering if there is a way of determining at compile time whether a class is an abstract base class or not? Yes. http://groups.google.com/groups?group=comp.lang.c%2B%2B.moderated&selm=df893da6.0207110613.75b2fe90%40posting.google.com Yoy will need GCC 3.4, VC 7.1, Intel C++ 7 or Comeau. is_abstract<> type traits will be available in next Boost release too. /Pavel [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Frank Birbacher Guest Posted: Wed Jan 21, 2004 9:14 am    Post subject: Re: Compile-time detection of abstract base classes Hi! J Lindrud wrote: Quote: ("error: can't instantiate abstract base class"). So in order to avoid this error, I'm wondering if there is a way of determining at compile time whether a class is an abstract base class or not? This error already occurs at compile-time, doesn't it? Frank [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top David Abrahams Guest Posted: Wed Jan 21, 2004 9:17 am    Post subject: Re: Compile-time detection of abstract base classes [email]jlindrud (AT) hotmail (DOT) com[/email] (J Lindrud) writes: Quote: Hi! If I have a type T& and I want to dereference it and create an object on the stack, ie "T t();", I run into problems when T happens to be an abstract base class ("error: can't instantiate abstract base class"). So in order to avoid this error, I'm wondering if there is a way of determining at compile time whether a class is an abstract base class or not? An is_abstract trait was recently submitted to Boost: http://tinyurl.com/3xqp2 I believe it is in the CVS now. -- Dave Abrahams Boost Consulting www.boost-consulting.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Hyman Rosen Guest Posted: Wed Jan 21, 2004 7:53 pm    Post subject: Re: Compile-time detection of abstract base classes J Lindrud wrote: Quote: create an object on the stack, ie "T t();" T t(); declares a function named 't' with no parameters returning a 'T'. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Joe Guest Posted: Fri Jan 23, 2004 10:00 am    Post subject: Re: Compile-time detection of abstract base classes Given a object oriented style of programming T& probably will be a reference to an abstract base. Give each class in your hierarchy a member function "virtual T clone()" to construct a copy of the class given only a reference or pointer to its abstract base. This is the "virtual constructor" idiom. It described in many C++ books. I know Scott Myers as a chapter on it. Everyone should have all of his C++ books. Joe "J Lindrud" <jlindrud (AT) hotmail (DOT) com> wrote Quote: Hi! If I have a type T& and I want to dereference it and create an object on the stack, ie "T t();", I run into problems when T happens to be an abstract base class ("error: can't instantiate abstract base class"). So in order to avoid this error, I'm wondering if there is a way of determining at compile time whether a class is an abstract base class or not? [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top J Lindrud Guest Posted: Sun Jan 25, 2004 1:32 am    Post subject: Re: Compile-time detection of abstract base classes Quote: Give each class in your hierarchy a member function "virtual T clone()" to construct a copy of the class given only a reference or pointer to its Only problem is that this is part of a serialization framework, that needs to work as-is with user-supplied classes, and it wouldn't be practical to require that the user always provide and properly implement a clone method. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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