C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Forward declare enums          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Russ Guest Posted: Thu Jun 16, 2005 8:13 am    Post subject: Forward declare enums Is there a portable way to forward declare enums? It seems that some compilers (e.g. VC++) allow it, and others don't. I need to fwd declare them but have it work with all compilers. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Me Guest Posted: Fri Jun 17, 2005 10:17 am    Post subject: Re: Forward declare enums Quote: Is there a portable way to forward declare enums? No. Quote: It seems that some compilers (e.g. VC++) allow it, and others don't. I need to fwd declare them but have it work with all compilers. In C, "enum foo;" is only allowed after foo has been defined. In C++, "enum foo;" isn't even allowed by the grammar. Compilers allow forward enum declarations as a (very welcome) extension. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Hyman Rosen Guest Posted: Fri Jun 17, 2005 10:23 am    Post subject: Re: Forward declare enums Russ wrote: Quote: Is there a portable way to forward declare enums? No, it is not standard-conforming to do so. There is also no need, since enums cannot take part in mutually recursive data structures, which is the only reason for forward declarations. [ 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: Fri Jun 17, 2005 1:16 pm    Post subject: Re: Forward declare enums "Russ" <cpp2319 (AT) hotmail (DOT) com> writes: Quote: Is there a portable way to forward declare enums? No. Quote: It seems that some compilers (e.g. VC++) allow it, and others don't. I need to fwd declare them but have it work with all compilers. Why do you need to declare an enum without defining it? [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ben Hutchings Guest Posted: Fri Jun 17, 2005 1:20 pm    Post subject: Re: Forward declare enums Russ <cpp2319 (AT) hotmail (DOT) com> wrote: Quote: Is there a portable way to forward declare enums? It seems that some compilers (e.g. VC++) allow it, and others don't. I need to fwd declare them but have it work with all compilers. No. The underlying integer type for an enumerated type can depend on the values of its enumerators. The representation of a pointer or reference to an enumerated type in turn can depend on that underlying integer type. So the language does not allow for incomplete enumerated types. -- Ben Hutchings Having problems with C++ templates? Your questions may be answered by <http://womble.decadentplace.org.uk/c++/template-faq.html>. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Torsten Robitzki Guest Posted: Fri Jun 17, 2005 1:36 pm    Post subject: Re: Forward declare enums Russ wrote: Quote: Is there a portable way to forward declare enums? It seems that some compilers (e.g. VC++) allow it, and others don't. I need to fwd declare them but have it work with all compilers. Then use an integer. regards Torsten [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Dave Harris Guest Posted: Fri Jun 17, 2005 1:38 pm    Post subject: Re: Forward declare enums [email]cpp2319 (AT) hotmail (DOT) com[/email] (Russ) wrote (abridged): Quote: Is there a portable way to forward declare enums? No. You can wrap them in a class and forward-declare that. struct Colour { enum Type { red, green, blue }; Colour( Type v=red ) : value(v) {} operator Colour() const { return value; } private: Type value; }; -- Dave Harris, Nottingham, UK. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top cookie__raver@hotmail.com Guest Posted: Fri Jun 17, 2005 1:40 pm    Post subject: Re: Forward declare enums Check out a recent column in Mathew Wilson's column in Dr Dobbs: http://www.ddj.com/columns/wilson/ Russ wrote: Quote: Is there a portable way to forward declare enums? It seems that some compilers (e.g. VC++) allow it, and others don't. I need to fwd declare them but have it work with all compilers. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Russ Guest Posted: Sat Jun 18, 2005 9:19 am    Post subject: Re: Forward declare enums Hyman Rosen wrote: Quote: Russ wrote: Is there a portable way to forward declare enums? No, it is not standard-conforming to do so. There is also no need, since enums cannot take part in mutually recursive data structures, which is the only reason for forward declarations. That's not so. Open your mind ... ;-) I am using (compiler-specific, via VC++ 2003) it for a good reaon, making library work for different programs in distributed sysstem with different data type ids [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top danny1066@hotmail.com Guest Posted: Sat Jun 18, 2005 9:19 am    Post subject: Re: Forward declare enums The STLSoft libraries provide a way that's portable, using a class wrapper and macros, like: // Forward declaration STLSOFT_DECLARE_FWD_ENUM(MessageCode); // Definition - can be diferent file STLSOFT_DEFINE_FWD_ENUM_BEGIN(MessageCode) mcEnter, mcExit, mcClear = 3, mcUnknown = -1, mcQuit = 4 STLSOFT_DEFINE_FWD_ENUM_END(MessageCode) under the covers its all a real enum, but the class is the thing that's forward declared, so it's entirely legal. You can even use it in a switch statement like a normal enum! The STLSoft documetnation's pretty bummed, as it is for all their libs, but the thing itself is really cool. We use STLSofts libs for lots of things, and just notiecd this had added in last release. One of my colleagues says Matt Wilsoin, who writes STLSoft libraries, wrote about it in a recent CUJ online aarticle, but havent seen it. Download it at http://synesis.com.au/software/stlsoft/downloads.html HTH D.T. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gene Bushuyev Guest Posted: Sat Jun 18, 2005 9:32 am    Post subject: Re: Forward declare enums <cookie__raver (AT) hotmail (DOT) com> wrote Quote: Check out a recent column in Mathew Wilson's column in Dr Dobbs: http://www.ddj.com/columns/wilson/ Which is not a good article. Had the author wrapped enum into a struct he wouldn't have too much to complain about: struct EnumType; // lo, forward declaration! Not really useful, of course. struct EnumType { enum type { first_const, second_const }; }; EnumType::type first_const() // look, no name clashing! { return EnumType::first_const; } Gene P.S. author's namespace wrapping is not a solution, it's a namespace misuse. P.P.S. author's using double underscores "__" and weird macros is telling quite a bit about the quality of the solutions presented in the articles. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Hyman Rosen Guest Posted: Tue Jun 21, 2005 10:00 am    Post subject: Re: Forward declare enums Russ wrote: Quote: That's not so. Open your mind ... ;-) I am using (compiler-specific, via VC++ 2003) it for a good reaon, making library work for different programs in distributed sysstem with different data type ids I'm just a simple programmer and your modern ways baffle and confuse me, but I know that there is no need to forward declare enums because you can just include the entire definition in the same spot. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Llewelly Guest Posted: Tue Jun 21, 2005 6:30 pm    Post subject: Re: Forward declare enums Hyman Rosen <hyrosen (AT) mail (DOT) com> writes: Quote: Russ wrote: That's not so. Open your mind ... ;-) I am using (compiler-specific, via VC++ 2003) it for a good reaon, making library work for different programs in distributed sysstem with different data type ids I'm just a simple programmer and your modern ways baffle and confuse me, but I know that there is no need to forward declare enums because you can just include the entire definition in the same spot. Some people like to provide users of a lib with only the enum's type, and keep the enumerators encapsulated where only the implementation can see them. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Hyman Rosen Guest Posted: Wed Jun 22, 2005 8:02 am    Post subject: Re: Forward declare enums Llewelly wrote: Quote: Some people like to provide users of a lib with only the enum's type, and keep the enumerators encapsulated where only the implementation can see them. But that's not possible in C++, because you cannot forward declare an enum. It's also impossible to "encapsulate" enum values, because they freely convert to integers, and then all values between 0 and one less than the next higher power of two of the converted value are legal to cast back to the enum type. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Wed Jun 22, 2005 1:11 pm    Post subject: Re: Forward declare enums In article <200506211840.j5LIe3Qd024100 (AT) cliffclavin (DOT) cs.rpi.edu>, Hyman Rosen <hyrosen (AT) mail (DOT) com> writes Quote: Llewelly wrote: Some people like to provide users of a lib with only the enum's type, and keep the enumerators encapsulated where only the implementation can see them. But that's not possible in C++, because you cannot forward declare an enum. It's also impossible to "encapsulate" enum values, because they freely convert to integers, and then all values between 0 and one less than the next higher power of two of the converted value are legal to cast back to the enum type. And I still find it difficult to understand why programmers who want (and often have good reasons for wanting) to 'forward' declare an enum find difficulty with: struct etype{ enum type{/* enumerators /*}; }; which can be declared with: struct etype; Even better, the enumerators are now properly scoped. -- Francis Glassborow ACCU Author of 'You Can Do It!' see http://www.spellen.org/youcandoit For project ideas and contributions: http://www.spellen.org/youcandoit/projects [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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