C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Name for a common idiom          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Gareth Stockwell Guest Posted: Sun Oct 24, 2004 4:13 am    Post subject: Name for a common idiom I've been using an technique whereby a tag used as a parameter to a template class C determines which of several potential base classes (A,B) C inherits from. What I want to know is whether there is an accepted name for this idiom. To illustrate the point, here is some code which demonstrates it in action... // Selector classes - these are typically just empty structs struct A_Tag { }; struct B_Tag { }; // Potential base classes class A { /* ... */ }; class B { /* ... */ }; // Definition of the class which selects the appropriate base for C template<class Tag> struct Selector { }; // Specialisations of the selector map tags onto base classes template<> struct Selector<A_Tag> { typedef A base_t; }; template<> struct Selector<B_Tag> { typedef B base_t; }; // OK, here is the class which we actually instantiate template<class Tag> class C : public Selector<Tag>::base_t { /* ... */ }; // And now the instantiations... C<A_Tag> c_inheriting_from_A; C<B_Tag> c_inheriting_from_B; Gareth [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Allen Yao Guest Posted: Tue Oct 26, 2004 4:24 am    Post subject: Re: Name for a common idiom Why not just: template <typename T> class C: public T { ... }; Quote: I've been using an technique whereby a tag used as a parameter to a template class C determines which of several potential base classes (A,B) C inherits from. What I want to know is whether there is an accepted name for this idiom. To illustrate the point, here is some code which demonstrates it in action... // Selector classes - these are typically just empty structs struct A_Tag { }; struct B_Tag { }; // Potential base classes class A { /* ... */ }; class B { /* ... */ }; // Definition of the class which selects the appropriate base for C template<class Tag> struct Selector { }; // Specialisations of the selector map tags onto base classes template<> struct Selector<A_Tag> { typedef A base_t; }; template<> struct Selector<B_Tag> { typedef B base_t; }; // OK, here is the class which we actually instantiate template<class Tag class C : public Selector { /* ... */ }; // And now the instantiations... C<A_Tag> c_inheriting_from_A; C<B_Tag> c_inheriting_from_B; Gareth [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Jonathan Turkanis Guest Posted: Tue Oct 26, 2004 4:24 am    Post subject: Re: Name for a common idiom "Gareth Stockwell" <gareth_stockwell (AT) hotmail (DOT) com> wrote Quote: I've been using an technique whereby a tag used as a parameter to a template class C determines which of several potential base classes (A,B) C inherits from. Why use a tag? Why not let the base class represent itself? Quote: // Definition of the class which selects the appropriate base for C template<class Tag> struct Selector { }; // Specialisations of the selector map tags onto base classes template<> struct Selector<A_Tag> { typedef A base_t; }; template<> struct Selector<B_Tag> { typedef B base_t; }; // OK, here is the class which we actually instantiate template<class Tag class C : public Selector { /* ... */ }; Why not: template<typename Base> class C : Base { /* ... */ }; ? Quote: // And now the instantiations... C<A_Tag> c_inheriting_from_A; C<B_Tag> c_inheriting_from_B; This becomes: C<A> // C inheriting from A C<B> // C inheriting from B If for some reason you need an extra level of indirection, inheriting from boost:mpl::if_ or boost:mpl::eval_if is much more flexible. Quote: Gareth Jonathan [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gareth Stockwell Guest Posted: Wed Oct 27, 2004 1:02 am    Post subject: Re: Name for a common idiom "Allen Yao" <yaozhen (AT) ustc (DOT) edu> wrote Quote: Why not just: template <typename T class C: public T { ... }; OK, my example was a trivial one, but typically the system is more complicated. For example, say we're writing a graph class, which consists of a number of components e.g. vertex container, edge container etc. I use the Composite pattern, inheriting Graph from the appropriate container objects - but I don't want the user to have to worry about the details of how those containers are implemented. In other words, I want to be able to write Graph< my_vertex_class, my_edge_class, VertexListTag, AdjacencyMatrixTag Quote: rather than Graph< my_vertex_class, my_edge_class, VertexListContainer AdjacencyMatrixContainer<my_edge_class> Quote: The user shouldn't *need* to know the name of the base class, or the parameters used to correctly define that base class. This allows me to decouple the definition of the Graph template from that of its component parts. Gareth [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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