omission of "virtual" in overridden method declarations in derived classes
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From: Edward Diener on 22 Feb 2010 09:51 Gregory Bok wrote: > Say you have a class with a virtual method, and then in a derived > class, the virtual method is declared without the "virtual" keyword. > > class A > { > public: > virtual void method(); > }; > > class B > { > public: > void method(); > }; > > Is this effectively an emulation of the "final" construct, e.g. this > becomes a leaf class in the absence of other overridable methods? No. > Does this impact the ability to treat instances of B polymorphically? No. > Or is the virtuality inherited implicitly from the root of the > inheritance tree even if it is omitted in intermediate classes? Yes. C++ does not require the programmer to repeat the 'virtual' keyword for a virtual method in a derived class. > Is > the behavior here implementation dependent? No. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Francis Glassborow on 22 Feb 2010 18:24 Gregory Bok wrote: > Say you have a class with a virtual method, and then in a derived > class, the virtual method is declared without the "virtual" keyword. > > class A > { > public: > virtual void method(); > }; > > class B > { > public: > void method(); > }; > > Is this effectively an emulation of the "final" construct, e.g. this > becomes a leaf class in the absence of other overridable methods? > Does this impact the ability to treat instances of B polymorphically? > Or is the virtuality inherited implicitly from the root of the > inheritance tree even if it is omitted in intermediate classes? Is > the behavior here implementation dependent? > Once a signature has been declared as virtual all derived implementaions are virtual. Indeed one problem for some programmers is that there is currently no way to remove the virtual qualification once it has been added by a base class. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Marco Nef on 22 Feb 2010 18:22 >> Or is the virtuality inherited implicitly from the root of the >> inheritance tree even if it is omitted in intermediate classes? > > Yes. C++ does not require the programmer to repeat the 'virtual' keyword > for a virtual method in a derived class. Which is one of the worst "features" of C++ in large projects, as it causes lots of errors (how do you know that a method is virtual if it is not written in the declaration of the class you are working with?). It should also be that base methods can only be called in the direct base class, or the programmer has to explicitly write a cast to skip a hierachy level. Some time ago I asked to change this in the new standard, so that virtual must be repeated in derived classes. But the reply in this group was that lots of old code would not compile anymore. That was a bad answer, as compilers could have a flag to ignore such an additional security feature in old code... - Marco -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Paul Bibbings on 23 Feb 2010 02:58 "Marco Nef" <maillist(a)shima.ch> writes: > Some time ago I asked to change this in the new standard, so that virtual > must be repeated in derived classes. But the reply in this group was that > lots of old code would not compile anymore. That was a bad answer, as > compilers could have a flag to ignore such an additional security feature in > old code... I would be happy even with the flip side of your suggestion - leaving the standard as is with regard to the virtual specifier yet having a compiler flag that could turn *on* a non-standard requirement that the virtual specifier be given everywhere, as an extension. Regards Paul Bibbings -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Paul Bibbings on 23 Feb 2010 02:58
Francis Glassborow <francis.glassborow(a)btinternet.com> writes: > Once a signature has been declared as virtual all derived implementaions > are virtual. Indeed one problem for some programmers is that there is > currently no way to remove the virtual qualification once it has been > added by a base class. I've always felt that this was a failing in the standard specification, not requiring the virtual qualification to be present in derived implementations. When faced with a large project code base it requires traversing a potentially deep class hierarchy right to the top to discover the virtual status of methods declared in a class that is being immediately subclassed. Of course there are other reasons that require the implementer of subclass to have full knowledge of a class hierarchy; information about an inherited interface, for example. However, in this instance, what seems odd is that the omission of the virtual specifier in an immediate base, where a method is in fact virtual, results in `partial information'. It might be supposed, carelessly, that if I have some information about an interface element, then I have *everything* I need to know about it. Regards Paul Bibbings -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] |