C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Problem with template using base class template in GCC          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Walt Karas Guest Posted: Fri Aug 27, 2004 3:11 am    Post subject: Problem with template using base class template in GCC The following gives an error in the declaration of the member function x() of the class template Tpl, compiliing with a recent version of GCC under Solaris: class A { }; class B { }; template <typename Base> class Tpl : protected Base { public: template <typename AOrB> int x(void) { return(baseX<AOrB>()); } }; class TheBase { public: template<typename AOrB> int baseX(void); }; template<> inline int TheBase::baseX<A>(void) { return(1); } template<> inline int TheBase::baseX<B>(void) { return(2); } Tpl<TheBase> aTpl; But if I give 'baseX' a dummy parameter whose type is the type parameter to the template, making explicit naming of the type paramenter in the baseX call unecessary, it does compile: class A { }; class B { }; template <typename Base> class Tpl : protected Base { public: template <typename AOrB> int x(void) { return(baseX(AOrB())); } }; class TheBase { public: template<typename AOrB> int baseX(AOrB aOrB); }; template<> inline int TheBase::baseX<A>(A a) { return(1); } template<> inline int TheBase::baseX<B>(B b) { return(2); } Tpl<TheBase> aTpl; Is there some rule in the standard that requires this behavior? Or is this just a problem with the GCC compiler? Another thing that surprised me was that GCC gives me an error if I put the partial specializations of 'baseX' inside the declaration of the class 'TheBase'. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gilles Rochefort Guest Posted: Sun Aug 29, 2004 11:15 am    Post subject: Re: Problem with template using base class template in GCC Quote: template inline int TheBase::baseX<A>(void) { return(1); } template inline int TheBase::baseX<B>(void) { return(2); } Tpl<TheBase> aTpl; But if I give 'baseX' a dummy parameter whose type is the type parameter to the template, making explicit naming of the type paramenter in the baseX call unecessary, it does compile: Well, I'm not sure to understand what you want to do here ???? But, from what you wrote, I guess that the compiler interpret the two specialized functions as two more functions in the class TheBase. Unfortunately, you don't explicitely make use of either class A or either class B,so the compiler can't decide wich one to choose. Quote: class TheBase { public: template<typename AOrB int baseX(AOrB aOrB); }; template inline int TheBase::baseX template inline int TheBase::baseX<B>(B b) { return(2); } Tpl<TheBase> aTpl; Here, as you provide two functions with different input arguments, compiler can now assign two functions to TheBase that are : int TheBase::baseX(A a); int TheBase::baseX(B b); This is a common overloading rule. Remember, you can't overload function with the same argument. Quote: Another thing that surprised me was that GCC gives me an error if I put the partial specializations of 'baseX' inside the declaration of the class 'TheBase'. Because of c++ doesn't allow template specializations inside a class ... More exactly specialized template should be associated with a namespace scope. Hope, all this will help ! Probably if you tell more about what you want to do exactly, I should help you a bit more. Best regards, Gilles Rochefort [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Chris Vine Guest Posted: Sun Aug 29, 2004 11:16 am    Post subject: Re: Problem with template using base class template in GCC Walt Karas wrote: Quote: The following gives an error in the declaration of the member function x() of the class template Tpl, compiliing with a recent version of GCC under Solaris: class A { }; class B { }; template <typename Base class Tpl : protected Base { public: template int x(void) { return(baseX ^^^^^^^^^^^^^^^^^^^^^^ (See below) Quote: }; class TheBase { public: template<typename AOrB int baseX(void); }; template inline int TheBase::baseX template inline int TheBase::baseX<B>(void) { return(2); } Tpl<TheBase> aTpl; [snip] I have had to change quite a lot of code of this kind to compile with gcc-3.4. According to gcc-3.4, the syntax for your call in int Tpl:() to a method in the templated base class is: int x(void) { return(Base::baseX<A0rB>()); } ^^^^^^ In other words, you need to specify that baseX() is a member of the templated base class. Whether that is required by the standard I know not, but I expect it is right. gcc-3.4 is a lot stricter than gcc-3.2/3.3. You have set follow-ups to a group I do not read, so I will not see any response (not a very good idea). Chris. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Paul Guest Posted: Sun Aug 29, 2004 10:40 pm    Post subject: Re: Problem with template using base class template in GCC <snip> Quote: Because of c++ doesn't allow template specializations inside a class ... More exactly specialized template should be associated with a namespace scope. Are you sure about that? class Test{ public: template<typename T>void foo(T){} template<>void foo(double){} }; The above compiles ok , not saying that means it's right, but are you ssaying this is not allowed? Is the namespace for foo here not Test? [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gilles Rochefort Guest Posted: Tue Aug 31, 2004 10:13 am    Post subject: Re: Problem with template using base class template in GCC Quote: class Test{ public: template<typename T>void foo(T){} template<>void foo(double){} }; Yes, as I said this not allowed ... The above code don't compile on a recent gcc ( I tested on a 3.3.2 ) I guess that gcc becomes lesser and lesser permissive .. So, now you have to write the following code to get a chance to compile. class Test{ public: template<typename T>void foo(T){} }; template<>void Test::foo(double){} Gilles. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Sharad Kala Guest Posted: Tue Aug 31, 2004 7:24 pm    Post subject: Re: Problem with template using base class template in GCC "Paul" <invisiblepaul (AT) hotmail (DOT) com> wrote Quote: snip Because of c++ doesn't allow template specializations inside a class ... More exactly specialized template should be associated with a namespace scope. Are you sure about that? class Test{ public: template<typename T>void foo(T){} template<>void foo(double){} }; Yes, he is correct. The relevant section in standard is 14.7.3/2: "An explicit specialization shall be declared in the namespace of which the template is a member, or, for member templates, in the namespace of which the enclosing class or enclosing class template is a member." -Sharad [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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