C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Virtual Constructors          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Thomas Matthews Guest Posted: Fri Dec 26, 2003 6:01 pm    Post subject: Virtual Constructors Hi, Is placing the keyword "virtual" in front of a constructor allowed as in the sample below? class TTable { virtual TTable(); }; My compiler, Borland Builder 5.2, has system libraries with this syntax. My understanding is that the virtual constructor is an idiom in which a virtual member function is called by the constructor. I didn't think that one could declare a constructor as virtual. -- Thomas Matthews C++ newsgroup welcome message: http://www.slack.net/~shiva/welcome.txt C++ Faq: http://www.parashift.com/c++-faq-lite C Faq: http://www.eskimo.com/~scs/c-faq/top.html alt.comp.lang.learn.c-c++ faq: http://www.raos.demon.uk/acllc-c++/faq.html Other sites: http://www.josuttis.com -- C++ STL Library book http://www.sgi.com/tech/stl -- Standard Template Library Back to top Jeff Schwab Guest Posted: Fri Dec 26, 2003 6:03 pm    Post subject: Re: Virtual Constructors Thomas Matthews wrote: Quote: Hi, Is placing the keyword "virtual" in front of a constructor allowed as in the sample below? No. Quote: class TTable { virtual TTable(); }; My compiler, Borland Builder 5.2, has system libraries with this syntax. My understanding is that the virtual constructor is an idiom in which a virtual member function is called by the constructor. I didn't think that one could declare a constructor as virtual. Back to top Ron Natalie Guest Posted: Fri Dec 26, 2003 6:08 pm    Post subject: Re: Virtual Constructors "Thomas Matthews" <Thomas_MatthewsSpamBotsSuck (AT) sbcglobal (DOT) net> wrote Quote: Hi, Is placing the keyword "virtual" in front of a constructor allowed as in the sample below? Of course not. What would it do? You sure it wasn't the destructor? Either that or it is some goofy Borland extension Back to top Cy Edmunds Guest Posted: Fri Dec 26, 2003 6:31 pm    Post subject: Re: Virtual Constructors "Thomas Matthews" <Thomas_MatthewsSpamBotsSuck (AT) sbcglobal (DOT) net> wrote in message news:3FEC7785.30806 (AT) sbcglobal (DOT) net... Quote: Hi, Is placing the keyword "virtual" in front of a constructor allowed as in the sample below? class TTable { virtual TTable(); }; My compiler, Borland Builder 5.2, has system libraries with this syntax. My understanding is that the virtual constructor is an idiom in which a virtual member function is called by the constructor. I didn't think that one could declare a constructor as virtual. You were right. Quote: -- Thomas Matthews C++ newsgroup welcome message: http://www.slack.net/~shiva/welcome.txt C++ Faq: http://www.parashift.com/c++-faq-lite C Faq: http://www.eskimo.com/~scs/c-faq/top.html alt.comp.lang.learn.c-c++ faq: http://www.raos.demon.uk/acllc-c++/faq.html Other sites: http://www.josuttis.com -- C++ STL Library book http://www.sgi.com/tech/stl -- Standard Template Library A constructor can't be virtual for the simple reason that when you are initializing an object you must know what the object is. However you can generate an object without knowing what type it is by a using a virtual function: class Base { public: virtual Base *clone() const = 0; virtual ~Base() {} }; class Derived : public Base { public: ... virtual Base *clone() const {return new Derived(*this);} }; A member function like clone() is sometimes called a virtual constructor. -- Cy http://home.rochester.rr.com/cyhome/ Back to top Thomas Matthews Guest Posted: Fri Dec 26, 2003 7:58 pm    Post subject: Re: Virtual Constructors Ron Natalie wrote: Quote: "Thomas Matthews" <Thomas_MatthewsSpamBotsSuck (AT) sbcglobal (DOT) net> wrote Hi, Is placing the keyword "virtual" in front of a constructor allowed as in the sample below? Of course not. What would it do? You sure it wasn't the destructor? Either that or it is some goofy Borland extension Here is the code from the header file (Includevcldbtables.hpp): class DELPHICLASS TTable; class PASCALIMPLEMENTATION TTable : public TDBDataSet { // snip -- irrelevant declarations public: __fastcall virtual TTable(Classes::TComponent* AOwner); __fastcall virtual ~TTable(void); // snip -- more declarations }; The above was cut and pasted directly from the file. I got a compilation error when I tried to create a descendant of TTable using a normal constructor. The error went away when I prefixed the declaration with "virtual" as the declaration above. This compiler would be so much better without all this nonstandard crap. -- Thomas Matthews C++ newsgroup welcome message: http://www.slack.net/~shiva/welcome.txt C++ Faq: http://www.parashift.com/c++-faq-lite C Faq: http://www.eskimo.com/~scs/c-faq/top.html alt.comp.lang.learn.c-c++ faq: http://www.raos.demon.uk/acllc-c++/faq.html Other sites: http://www.josuttis.com -- C++ STL Library book http://www.sgi.com/tech/stl -- Standard Template Library Back to top John Carson Guest Posted: Sat Dec 27, 2003 1:14 am    Post subject: Re: Virtual Constructors "Thomas Matthews" <Thomas_MatthewsSpamBotsSuck (AT) sbcglobal (DOT) net> wrote in message news:3FEC7785.30806 (AT) sbcglobal (DOT) net Quote: Hi, Is placing the keyword "virtual" in front of a constructor allowed as in the sample below? class TTable { virtual TTable(); }; My compiler, Borland Builder 5.2, has system libraries with this syntax. My understanding is that the virtual constructor is an idiom in which a virtual member function is called by the constructor. I didn't think that one could declare a constructor as virtual. You can't have a virtual constructor. See Stroustrup TC++PL, p. 424-5. -- John Carson 1. To reply to email address, remove donald 2. Don't reply to email address (post here instead) Back to top Darko Miletic Guest Posted: Sun Jan 04, 2004 11:43 am    Post subject: Re: Virtual Constructors On Fri, 26 Dec 2003 19:58:53 GMT, Thomas Matthews <Thomas_MatthewsSpamBotsSuck (AT) sbcglobal (DOT) net> wrote: Quote: Here is the code from the header file (Includevcldbtables.hpp): class DELPHICLASS TTable; class PASCALIMPLEMENTATION TTable : public TDBDataSet { // snip -- irrelevant declarations public: __fastcall virtual TTable(Classes::TComponent* AOwner); __fastcall virtual ~TTable(void); // snip -- more declarations }; Here is an excerpt from C++ Builder help: "The delphiclass argument is used for declarations for classes derived from TObject. These classes will be created with the following VCL compatibility: VCL-compatible RTTI VCL-compatible constructor/destructor behavior VCL-compatible exception handling A VCL-compatible class has the following restrictions. No virtual base classes are allowed. No multiple inheritance is allowed. Must be dynamically allocated by using the global new operator. Must have a destructor. Copy constructors and assignment operators are not compiler-generated for VCL-derived classes. A class declaration that is translated from Object Pascal will need this modifier if the compiler needs to know that the class is derived from TObject." So in short this is special extension that enables C++ Builder to use VCL which, of course, is written in Delphi and is not intended to be used in any other way. Quote: This compiler would be so much better without all this nonstandard crap. You should not use this extension as it is related to VCL only. And without that "nonstandard crap" there would be no easy VCL usage. And without that C++ Builder would not be very usable. 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