C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  template template parameters using templatized nested classe          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Dave B Guest Posted: Sat Sep 25, 2004 10:24 am    Post subject: template template parameters using templatized nested classe template template parameters using templatized nested classes I'm getting a bunch of different results from multiple compilers when using (or rather attempting to use) a templatized nested class (of a templatized outer class) as a default template template parameter. The following are some brief examples of the problems I've been encountering. The questions I'm hoping to have answered are: What is the proper way to accomplish what is implied in the following examples? What (if any) is the Standard's response? And in regards to the Standard, which compiler is doing it right? A quick note... you'll notice that I'm using ver 2.95 of the GCC compiler in my examples. I'm sure some of these issues would be solved with a newer version. However, I can't use a newer version due to the limitations of the project I'm on (Playstation 2 development). I would be curious to see how newer versions of the GCC compiler would handle it. EXAMPLE 1 --------- /** * This structure will be referenced in each example...I'm * just listing it once for brevity. */ template <typename T1, typename T2> struct Outer { template <typename T3> struct Inner { }; }; template < typename T1, typename T2, template class TCls = Outer<int, int>::Inner // notice explicit template parameters <int, int> Quote: struct MyTest : public TCls<T1> { }; //////////////////// Results //////////////////////// gcc 2.95 - Internal compiler error Intel 8.0 - Success MSVC 7.1 - Success MSVC 8.0 Beta - Success //////////////////////////////////////////////////// EXAMPLE 2 --------- If I define the default template template parameter to be the inner class of an outer templatized class defined using template parameters previously declared in the template argument list... template < typename T1, typename T2, template class TCls = Outer<T1, T2>::Inner // notice use of template argments <T1, T2> Quote: struct MyTest : public TCls<T1> { }; //////////////////// Results //////////////////////// gcc 2.95 - Internal compiler error Intel 8.0 - error: nontype "Outer<T1, T2>::Inner [with T1=T1, T2=T2]" is not a class template MSVC 7.1 - error C3202: 'Inner' : invalid default argment for template parameter '<Unknown>', expected a class template MSVC 8.0 Beta - Same as 7.1 ///////////////////////////////////////////////////// EXAMPLE 3 --------- However, the following works without a problem (well, except for gcc)... template < typename T1, typename T2 Quote: struct MyTest : public Outer indirect use of template template parameter as base class //////////////////// Results //////////////////////// gcc 2.95 - Internal compiler error Intel 8.0 - Success MSVC 7.1 - Success MSVC 8.0 Beta - Success ///////////////////////////////////////////////////// EXAMPLE 4 --------- I read in a usenet post somewhere that using the 'template' keyword when using a templatized dependent name (like the templatized nested class) can help remedy the problem. The use of 'template' is similar to 'typename' (when specifying that a dependent name is a type) in that you are telling the compiler that the dependent name is a template. So, I tried... template < typename T1, typename T2, template class TCls = Outer<T1, T2>::template Inner // notice the 'template' keyword qualifying 'Inner' Quote: struct MyTest : public TCls<T1> { }; //////////////////// Results //////////////////////// gcc 2.95 - Internal compiler error Intel 8.0 - Success MSVC 7.1 - 'Inner' : invalid default argument for template parameter '<Unknown>', expected a class template. MSVC 8.0 Beta - Same as 7.1 ///////////////////////////////////////////////////// EXAMPLE 5 --------- So, it seems that the MSVC compiler doesn't like (or just ignores) the use of 'template' as indicated above. But, the following works fine (similar to Example 3 above)... template < typename T1, typename T2 Quote: struct MyTest : public Outer //////////////////// Results //////////////////////// gcc 2.95 - Success (Even GCC gets it right!) Intel 8.0 - Success MSVC 7.1 - Success MSVC 8.0 Beta - Success ////////////////////////////////////////////////////// [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Glen Low Guest Posted: Sun Sep 26, 2004 9:53 am    Post subject: Re: template template parameters using templatized nested cl Quote: I'm getting a bunch of different results from multiple compilers when using (or rather attempting to use) a templatized nested class (of a templatized outer class) as a default template template parameter. I too have had similar difficulties with the use of template template parameters. gcc seems to be the best so far, even in places which require the "template" keyword to signify a template member. xlc++ seems a little more conformant, and Metrowerks tends to react poorly to partial specializations involving nested templates. Quote: The following are some brief examples of the problems I've been encountering. The questions I'm hoping to have answered are: What is the proper way to accomplish what is implied in the following examples? What (if any) is the Standard's response? And in regards to the Standard, which compiler is doing it right? I'll only comment on your use of the "template" keyword, see 14.2/4: When the name of a member template specialization appears after . or -> in a postfix-expression, or after nested-name-specifier in a qualified-id, and the postfix-expression or qualified-id explicitly depends on a template-parameter (temp.dep), the member template name must be prefixed by the keyword template. Otherwise the name is assumed to name a non-template. i.e. some expression of the sort a.b or a->b or a::b, that depends on a template parameter, must be expressed as a. template b or a->template b or a::template b. I'd personally stay away from template template params that are nested templates because of these problems. A better solution might be to use a template type parameter instead, and use the nested template within that. Cheers, Glen Low, Pixelglow Software www.pixelglow.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Alberto Barbati Guest Posted: Mon Sep 27, 2004 9:08 pm    Post subject: Re: template template parameters using templatized nested cl I completely agree with Glen. Quote: I'd personally stay away from template template params that are nested templates because of these problems. A better solution might be to use a template type parameter instead, and use the nested template within that. .....or even going all the way to bottom, as in: template < typename T1, typename T2, class Base = typename Outer Quote: struct MyTest : public Base { }; The only good reason to use the template parameter is if you need to instantiate the template with a different type in the implementation of MyTest. You could still obtain that using a rebind hack like std::allocator does. Although less elegant, it should compile like a charm even on PS2... ;-) Alberto [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Dave B Guest Posted: Tue Sep 28, 2004 10:19 am    Post subject: Re: template template parameters using templatized nested cl Quote: The only good reason to use the template parameter is if you need to instantiate the template with a different type in the implementation of MyTest. Unfortunately, that's exactly what I need it for...although the example I posted doesn't reflect that need (I just used it as a simple test case). Quote: You could still obtain that using a rebind hack like std::allocator does. I will look into the rebind hack (I actually considered it as an option before) as you recommend. Upon further investigation, I've concluded that the proper usage as supported by the Standard is indeed: template < typename T1, typename T2, template Quote: struct MyTest : public TCls<T1> { }; The sections of interest are: Section 14.1.9 - "A default template-argument may be specified for any kind of template-parameter (type, non-type, template)." The TCls parameter above is a template template-parameter. It can have a default template-argument. Section 14.1.13 - "The scope of a template-parameter extends from its point of declaration until the end of its template. In particular, a template-parameter can be used in the declaration of subsequent template-parameters and their default arguments." Template parameters T1 and T2 were declared prior to using them in the declaration of the default template-argument. Section 14.2.4 (Thanks Glen!) - "When the name of a member template specialization appears after . or -> in a postfix-expression, or after nested-name-specifier in a qualified-id, and the postfix-expression or qualifie-id explicitly depends on a template-parameter, the member template name must be prefixed by the keyword template. Otherwise the name is assumed to name a non-template." The template keyword is needed to prefix 'Inner' as it is a dependent member template of Outer. I've submitted a report with the Visual Studio team to inform them that their compilers (8.0 Beta and 7.1...haven't checked other versions) are not compliant in this respect. As for GCC ver. 2.95 for Playstation 2... well, I switched to the SN Systems PS2 Compiler instead. Everything compiled fine! I appreciate everyones help. -Dave [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Glen Low Guest Posted: Wed Sep 29, 2004 9:40 am    Post subject: Re: template template parameters using templatized nested cl Quote: Upon further investigation, I've concluded that the proper usage as supported by the Standard is indeed: template typename T1, typename T2, template <typename> class TCls = Outer<T1, T2>::template Inner struct MyTest : public TCls<T1> { }; Section 14.2.4 (Thanks Glen!) - "When the name of a member template specialization appears after . or -> in a postfix-expression, or after nested-name-specifier in a qualified-id, and the postfix-expression or qualifie-id explicitly depends on a template-parameter, the member template name must be prefixed by the keyword template. Otherwise the name is assumed to name a non-template." The template keyword is needed to prefix 'Inner' as it is a dependent member template of Outer. Not so fast, Dave. The section says "when the name of a member template specialization" but Inner by itself is not a specialization. Therefore I think it should be template <typename> class TCls = Outer <T1, T2>::Inner Cheers, Glen Low, Pixelglow Software www.pixelglow.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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