C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Templates, Typedefs, Structs....          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Let_Me_Be Guest Posted: Sun Sep 03, 2006 8:14 pm    Post subject: Templates, Typedefs, Structs.... Hi, sorry for the Subject, I'm currently having a big problem in my project. It has a very strict architecture, and I'm geting into problems with types like this: Pointer< StaticArray<size, StaticArray<size, Pointer< VariableArray< Pointer< Item<size> > >.... as you can see, the type is realy long, so I created something like this: template < uint size > class BaseTypeDefiner { public: typedef Item<size> Item; typedef Pointer<Item> PItem; ..... }; Then using BaseTypeDefiner<size>::Instead of the long one above. The problem is that using BaseTypeDefiner<2>::Item works just fine, but template < uint size> class MyClass { public: BaseTypeDefiner<size>::Item item; }; throw an error - BaseTypeDefiner<size>::Item is not a type. What am I doing wrong? Thx for help... [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Jens Theisen Guest Posted: Mon Sep 04, 2006 6:32 pm    Post subject: Re: Templates, Typedefs, Structs.... Let_Me_Be wrote: Quote: What am I doing wrong? Try public: typename BaseTypeDefiner<size>::Item item; It's similar to the FAQ http://www.parashift.com/c++-faq-lite/templates.html#faq-35.18 Unfortuately, within a template, you have to help the parser and tell him that BaseTypeDefiner<size>::Item is a type, because at that point it can't tell, but needs to tell in order to continue parsing. Jens [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Hrayr BABAJANYAN Guest Posted: Mon Sep 04, 2006 6:34 pm    Post subject: Re: Templates, Typedefs, Structs.... Hi, Let_Me_Be wrote: Quote: Hi, sorry for the Subject, I'm currently having a big problem in my project. It has a very strict architecture, and I'm geting into problems with types like this: Pointer< StaticArray<size, StaticArray<size, Pointer< VariableArray< Pointer< Item<size> > >.... as you can see, the type is realy long, so I created something like this: template < uint size > class BaseTypeDefiner { public: typedef Item<size> Item; typedef Pointer<Item> PItem; .... }; Then using BaseTypeDefiner<size>::Instead of the long one above. The problem is that using BaseTypeDefiner<2>::Item works just fine, but template < uint size> class MyClass { public: BaseTypeDefiner<size>::Item item; }; throw an error - BaseTypeDefiner<size>::Item is not a type. What am I doing wrong? Thx for help... Well, the compiler does not know whether the BaseTypeDefiner<size>::Item is a type or not, some compilers do not support `implicit typename' (in other it can be deprecated), so you basicly need to explicitly specify that the BaseTypeDefiner<size>::Item is a typename. [see the code below] template <uint size> class MyClass { public: typename BaseTypeDefiner<size>::Item item; }; Cheers! -- Hrayr [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Maeder Guest Posted: Mon Sep 04, 2006 6:34 pm    Post subject: Re: Templates, Typedefs, Structs.... "Let_Me_Be" <Happy.Cerberus (AT) gmail (DOT) com> writes: Quote: template < uint size > class BaseTypeDefiner { public: typedef Item<size> Item; typedef Pointer<Item> PItem; .... }; Then using BaseTypeDefiner<size>::Instead of the long one above. The problem is that using BaseTypeDefiner<2>::Item works just fine, but template < uint size> class MyClass { public: BaseTypeDefiner<size>::Item item; }; throw an error - BaseTypeDefiner<size>::Item is not a type. The compiler has to figure out the meaning of the name Item in BaseTypeDefiner<size>::Item. This is a dependant name; a later specialization of MyClass could define Item to something different than the base template. For dependant names, the compiler has to assume that they do *not* name a type unless the code explicitly says so, as in: typename BaseTypeDefiner<size>::Item item; [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Jens Kilian Guest Posted: Mon Sep 04, 2006 6:35 pm    Post subject: Re: Templates, Typedefs, Structs.... "Let_Me_Be" <Happy.Cerberus (AT) gmail (DOT) com> writes: Quote: The problem is that using BaseTypeDefiner<2>::Item works just fine, but template < uint size> class MyClass { public: BaseTypeDefiner<size>::Item item; }; throw an error - BaseTypeDefiner<size>::Item is not a type. What am I doing wrong? You forgot the 'typename' keyword, which tells the compiler that BaseTypeDefiner<size>::Item *is* a type. template <uint size> class MyClass { public: typename BaseTypeDefiner<size>::Item item; };// ^^^^^^^^ HTH, Jens. -- mailto:jjk (AT) acm (DOT) org As the air to a bird, or the sea to a fish, http://www.bawue.de/~jjk/ so is contempt to the contemptible. [Blake] http://del.icio.us/jjk [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top John Moeller Guest Posted: Mon Sep 04, 2006 6:37 pm    Post subject: Re: Templates, Typedefs, Structs.... Let_Me_Be wrote: Quote: Hi, sorry for the Subject, I'm currently having a big problem in my project. It has a very strict architecture, and I'm geting into problems with types like this: Pointer< StaticArray<size, StaticArray<size, Pointer< VariableArray< Pointer< Item<size> > >.... as you can see, the type is realy long, so I created something like this: template < uint size > class BaseTypeDefiner { public: typedef Item<size> Item; typedef Pointer<Item> PItem; .... }; Then using BaseTypeDefiner<size>::Instead of the long one above. The problem is that using BaseTypeDefiner<2>::Item works just fine, but template < uint size> class MyClass { public: BaseTypeDefiner<size>::Item item; }; throw an error - BaseTypeDefiner<size>::Item is not a type. What am I doing wrong? Item is a dependent name of another template, in this case, BaseTypeDefinter<>, so it needs to be qualified with the "typename" keyword, like this: template <uint size> class MyClass { public: // needs "typename" here: typename BaseTypeDefiner<size>::Item item; }; There can be ambiguities with dependent type names, because the compiler often can't determine whether a name is a static member variable, a member function, a type, etc., so you need to qualify every dependent name with "typename" if you use it as a type. If you think that's unwieldy, you can always typedef it again if you want to be able to use it repeatedly: template <uint size> class MyClass { public: typedef typename BaseTypeDefiner<size>::Item Item; Item item; }; [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Tutone Guest Posted: Mon Sep 04, 2006 6:37 pm    Post subject: Re: Templates, Typedefs, Structs.... Let_Me_Be wrote: Quote: Hi, sorry for the Subject, I'm currently having a big problem in my project. It has a very strict architecture, and I'm geting into problems with types like this: Pointer< StaticArray<size, StaticArray<size, Pointer< VariableArray< Pointer< Item<size> > >.... as you can see, the type is realy long, so I created something like this: template < uint size > class BaseTypeDefiner { public: typedef Item<size> Item; typedef Pointer<Item> PItem; .... }; Then using BaseTypeDefiner<size>::Instead of the long one above. The problem is that using BaseTypeDefiner<2>::Item works just fine, but template < uint size> class MyClass { public: BaseTypeDefiner<size>::Item item; Change the above line to: typename BaseTypeDefiner<size>::Item item; Best regards, Tom [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ivan Vecerina Guest Posted: Mon Sep 04, 2006 6:37 pm    Post subject: Re: Templates, Typedefs, Structs.... "Let_Me_Be" <Happy.Cerberus (AT) gmail (DOT) com> wrote in message news:1157280550.032652.324660 (AT) e3g2000cwe (DOT) googlegroups.com... : template < uint size > class BaseTypeDefiner : { : public: : typedef Item<size> Item; : typedef Pointer<Item> PItem; : .... : }; : : Then using BaseTypeDefiner<size>::Instead of the long one above. : : The problem is that using BaseTypeDefiner<2>::Item works just fine, but : template < uint size> class MyClass : { : public: : BaseTypeDefiner<size>::Item item; : }; : throw an error - BaseTypeDefiner<size>::Item is not a type. : : What am I doing wrong? You need to use the 'typename' keyword to disambiguate references to some identifiers within template scope. In the last class above, you need to write: typename BaseTypeDefiner<size>::Item item; There is a technical reason why this is so: when interpreting the definition of the MyClass template, the compiler cannot know whether BaseTypeDefiner<size>::Item is a type or another kind of member, and this would lead to some ambiguities. 'typename' tells the compiler that the referenced identifier is a type. {However please look at the settings on your news-reader because I had to manually delete double line spacing. -mod} hth -Ivan -- http://ivan.vecerina.com/contact/?subject=NG_POST <- email contact form Brainbench MVP for C++ <> http://www.brainbench.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ismail Pazarbasi Guest Posted: Mon Sep 04, 2006 6:40 pm    Post subject: Re: Templates, Typedefs, Structs.... Let_Me_Be wrote: Quote: Hi, sorry for the Subject, I'm currently having a big problem in my project. It has a very strict architecture, and I'm geting into problems with types like this: Pointer< StaticArray<size, StaticArray<size, Pointer< VariableArray< Pointer< Item<size> > >.... as you can see, the type is realy long, so I created something like this: template < uint size > class BaseTypeDefiner { public: typedef Item<size> Item; typedef Pointer<Item> PItem; .... }; Then using BaseTypeDefiner<size>::Instead of the long one above. The problem is that using BaseTypeDefiner<2>::Item works just fine, but template < uint size> class MyClass { public: BaseTypeDefiner<size>::Item item; }; throw an error - BaseTypeDefiner<size>::Item is not a type. What am I doing wrong? Thx for help... BaseTypeDefiner<T>::Item is a dependant name. Therefore, you should add "typename" before BaseTypeDefiner<T> in MyClass. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top AlbertSSj Guest Posted: Tue Sep 05, 2006 6:54 am    Post subject: Re: Templates, Typedefs, Structs.... Quote: template < uint size> class MyClass { public: BaseTypeDefiner<size>::Item item; }; throw an error - BaseTypeDefiner<size>::Item is not a type. Probably a typename can fix this. So I'll rewrite like this: template < uint size> class MyClass { public: typedef typename BaseTypeDefiner<size>::Item Item; // My preferred way Item item; }; Another solution (not the best since you have to rewrite typename all the times) is template < uint size> class MyClass { public: typedef typename BaseTypeDefiner<size>::Item item; // I dont like this one a lot }; -- An update... I tried this on gcc 3.3 and compile, so this should be the solution ;) [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top James Dennett Guest Posted: Tue Sep 05, 2006 5:34 pm    Post subject: Re: Templates, Typedefs, Structs.... AlbertSSj wrote: Quote: template < uint size> class MyClass { public: BaseTypeDefiner<size>::Item item; }; throw an error - BaseTypeDefiner<size>::Item is not a type. Probably a typename can fix this. So I'll rewrite like this: template < uint size> class MyClass { public: typedef typename BaseTypeDefiner<size>::Item Item; // My preferred way Item item; }; Another solution (not the best since you have to rewrite typename all the times) is template < uint size> class MyClass { public: typedef typename BaseTypeDefiner<size>::Item item; // I dont like this one a lot }; I think the "typedef" in that last example should be deleted, as the idea was to define a data member, not a typedef. -- James [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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