C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Question about elaborate CRTP - is the friend declaration i          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Mikhail Parakhin Guest Posted: Sun Sep 10, 2006 11:12 pm    Post subject: Question about elaborate CRTP - is the friend declaration i { Extranous blank lines (double-spacing) removed. -mod } I'm using the following form of CRTP: template<class P_This> class T_Policy { protected: void F() {static_cast<P_This*>(this)->G();} }; template<template<class> class P_Policy> class T_MyClass : public P_Policy<T_MyClass<P_Policy> > { friend class P_Policy<T_MyClass<P_Policy> >; private: void G() {} }; Reason is that I want to give the user of T_MyClass an ability to change policies and still don't want any virtual function overhead. Now, my question is, does class P_Policy<T_MyClass<P_Policy> > constitute an "elaborated type specifier"? In other words, can I actually use the friend declaration like this? Because normal class-type template parameters cannot be used in friend declaration. Mikhail Parakhin [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gennaro Prota Guest Posted: Mon Sep 11, 2006 4:17 am    Post subject: Re: Question about elaborate CRTP - is the friend declaratio On 10 Sep 2006 14:12:20 -0400, "Mikhail Parakhin" <parakhin (AT) comcast (DOT) net> wrote: Quote: I'm using the following form of CRTP: template<class P_This class T_Policy { protected: void F() {static_cast<P_This*>(this)->G();} }; template<template<class> class P_Policy class T_MyClass : public P_Policy<T_MyClass<P_Policy { friend class P_Policy<T_MyClass<P_Policy> >; private: void G() {} }; Reason is that I want to give the user of T_MyClass an ability to change policies and still don't want any virtual function overhead. Now, my question is, does class P_Policy<T_MyClass<P_Policy constitute an "elaborated type specifier"? Yes, but the friend declaration is still well-formed C++03 :-) In fact, the rule (relaxed in the current working paper) is that the form "friend class typedef-name", which is the same as "friend class identifier", is illegal, not any elaborated-type-specifier. -- Gennaro Prota [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Mikhail Parakhin Guest Posted: Mon Sep 11, 2006 6:29 pm    Post subject: Re: Question about elaborate CRTP - is the friend declaratio Gennaro, 11.4/2 says: "An elaborated-type-specifier shall be used in a friend declaration". So, ONLY elaborated-type-specifiers are permitted. I know that template<class T> class C { friend class T; }; is illigal, because class T is ill-formed. What I'm trying to understand is whether friend class P_Policy<T_MyClass<P_Policy> >; is well formed elaborated type specifier. It seams, that in case of template template parameter it should be well-formed, and MSVC 8.0 compiles it - but I want to be sure. Mikhail [I do not want replies, please followup to the group.] "Gennaro Prota" <gennaro_prota (AT) yahoo (DOT) com> wrote in message news:av09g2hk2o42o2oo6db04nir4vsitetnu8 (AT) 4ax (DOT) com... Quote: On 10 Sep 2006 14:12:20 -0400, "Mikhail Parakhin" parakhin (AT) comcast (DOT) net> wrote: I'm using the following form of CRTP: template<class P_This class T_Policy { protected: void F() {static_cast<P_This*>(this)->G();} }; template<template<class> class P_Policy class T_MyClass : public P_Policy<T_MyClass<P_Policy { friend class P_Policy<T_MyClass<P_Policy> >; private: void G() {} }; Reason is that I want to give the user of T_MyClass an ability to change policies and still don't want any virtual function overhead. Now, my question is, does class P_Policy<T_MyClass<P_Policy constitute an "elaborated type specifier"? Yes, but the friend declaration is still well-formed C++03 :-) In fact, the rule (relaxed in the current working paper) is that the form "friend class typedef-name", which is the same as "friend class identifier", is illegal, not any elaborated-type-specifier. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gennaro Prota Guest Posted: Tue Sep 12, 2006 2:01 am    Post subject: Re: Question about elaborate CRTP - is the friend declaratio On 11 Sep 2006 09:29:45 -0400, "Mikhail Parakhin" <clcppm-poster (AT) this (DOT) is.invalid> wrote: Quote: Gennaro, 11.4/2 says: "An elaborated-type-specifier shall be used in a friend declaration". So, ONLY elaborated-type-specifiers are permitted. Incidentally I can't find that in the current working paper, but that's not the point. Quote: I know that template<class T class C { friend class T; }; is illigal, because class T is ill-formed. No, "class T" is not ill-formed, "friend class T" is. See 7.1.5.3/2. Quote: What I'm trying to understand is whether friend class P_Policy<T_MyClass<P_Policy> >; is well formed elaborated type specifier. It seams, that in case of template template parameter it should be well-formed, and MSVC 8.0 compiles it - but I want to be sure. I believe it is valid. I can't see any sentence analogous to the one in 7.1.5.3/2 to make it ill-formed (and if such a sentence existed why not putting it in the same section --but the standard does have such oddities, where logically related statements are scattered in different places without explicit links). -- Gennaro Prota [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gennaro Prota Guest Posted: Tue Sep 12, 2006 2:01 am    Post subject: Re: Question about elaborate CRTP - is the friend declaratio On 10 Sep 2006 19:17:00 -0400, Gennaro Prota <gennaro_prota (AT) yahoo (DOT) com> wrote: Quote: On 10 Sep 2006 14:12:20 -0400, "Mikhail Parakhin" parakhin (AT) comcast (DOT) net> wrote: Now, my question is, does class P_Policy<T_MyClass<P_Policy constitute an "elaborated type specifier"? Yes, but the friend declaration is still well-formed C++03 :-) In fact, the rule (relaxed in the current working paper) is that the form "friend class typedef-name", which is the same as "friend class identifier", is illegal, not any elaborated-type-specifier. Some sloppy editing here, sorry. I meant: "the form 'friend class identifier' is illegal when identifier resolves to a typedef-name. Other forms, such as the one in the OP example, are valid elaborated-type-specifiers". -- Gennaro Prota [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Alberto Ganesh Barbati Guest Posted: Thu Sep 14, 2006 5:15 pm    Post subject: Re: Question about elaborate CRTP - is the friend declaratio Gennaro Prota ha scritto: Quote: No, "class T" is not ill-formed, "friend class T" is. See 7.1.5.3/2. 7.1.5.3/2 says "If the identifier resolves to a typedef-name or a template type-parameter, the elaborated-type-specifier is ill-formed." So I think the OP is right, the elaborated-type-specifier "class T" is ill-formed, and that in turn makes the declaration "friend class T" ill-formed as well. Quote: What I'm trying to understand is whether friend class P_Policy<T_MyClass<P_Policy> >; is well formed elaborated type specifier. It seams, that in case of template template parameter it should be well-formed, and MSVC 8.0 compiles it - but I want to be sure. I believe it is valid. I can't see any sentence analogous to the one in 7.1.5.3/2 to make it ill-formed (and if such a sentence existed why not putting it in the same section --but the standard does have such oddities, where logically related statements are scattered in different places without explicit links). P_Policy<T_MyClass<P_Policy> > is a template-id (14.2/1) so 7.1.5.3/1 explicitly says the declaration is s well-formed (it's the third case). The fact that we have template template parameters is irrelevant, even with non-template template parameters it would be ok. Ganesh [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Mikhail Parakhin Guest Posted: Thu Sep 14, 2006 9:49 pm    Post subject: Re: Question about elaborate CRTP - is the friend declaratio Quote: 7.1.5.3/2 says "If the identifier resolves to a typedef-name or a template type-parameter, the elaborated-type-specifier is ill-formed." So I think the OP is right, the elaborated-type-specifier "class T" is ill-formed, and that in turn makes the declaration "friend class T" ill-formed as well. I totally agree. That was my understanding as well Quote: The fact that we have template template parameters is irrelevant, even with non-template template parameters it would be ok. I don't quite understand what you mean. My understanding is that template template parameter is the gist here. If it wasn't template template parameter, it would be something like template <class P_Policy> class T_MyClass { friend class P_Policy; // Illegal - resolves to "template type-parameter"! }; With template template parameter it is template <template<class >class P_Policy> class T_MyClass { friend class P_Policy<T_MyClass<P_Policy> >; // seams to be legal, because it is not a "template type-parameter", but "template template parameter". }; Mikhail Parakhin [I do not want replies, please followup to the group.] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Alberto Ganesh Barbati Guest Posted: Sat Sep 16, 2006 5:15 am    Post subject: Re: Question about elaborate CRTP - is the friend declaratio Mikhail Parakhin ha scritto: Quote: The fact that we have template template parameters is irrelevant, even with non-template template parameters it would be ok. I don't quite understand what you mean. My understanding is that template template parameter is the gist here. If it wasn't template template parameter, it would be something like template <class P_Policy class T_MyClass { friend class P_Policy; // Illegal - resolves to "template type-parameter"! }; Oh, now I understand your point. That's illegal, but not because "P_Policy" is a *type* parameter, but simply because it is the name of a parameter *used alone*. But if the parameter is used as part of a template-id, then it's legal: template <template<class >class P_Policy> class T_MyClass { friend class P_Policy<T_MyClass<P_Policy> >; // legal! }; template <class P_Policy> class T_MyClass { friend class AnyTemplate<P_Policy>; // legal! }; They're both legal because "P_Policy<T_MyClass<P_Policy> >" and "AnyTemplate<P_Policy>" are template-ids. Ganesh [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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