C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  typeid operator Goto page 1, 2  Next          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Marco Jez Guest Posted: Wed Oct 27, 2004 6:29 pm    Post subject: typeid operator Hi everyone! I would like to use the reference returned by typeid as key in a std::map. Is it safe to assume that typeid(T) (where T is a type name) will always return the same reference to the same type_info structure for a given T? My map would look like this: typedef std::map<const std::type_info &, ......> Type_map; Cheers, Marco Back to top Andrew Koenig Guest Posted: Wed Oct 27, 2004 7:16 pm    Post subject: Re: typeid operator "Marco Jez" <nessuno (AT) nessuno (DOT) com> wrote Quote: I would like to use the reference returned by typeid as key in a std::map. Is it safe to assume that typeid(T) (where T is a type name) will always return the same reference to the same type_info structure for a given T? I don't see why you should be able to make that assumption. The type_info::before member is intended to be used for defining a comparison function that you can use for making map keys out of typeids. If you use it, two distinct type_info objects that represent the same type should be treated as equivalent keys. Back to top Victor Bazarov Guest Posted: Wed Oct 27, 2004 7:18 pm    Post subject: Re: typeid operator Marco Jez wrote: Quote: I would like to use the reference returned by typeid as key in a std::map. Is it safe to assume that typeid(T) (where T is a type name) will always return the same reference to the same type_info structure for a given T? My map would look like this: typedef std::map<const std::type_info &, ......> Type_map; Yes, that's guaranteed. V Back to top Victor Bazarov Guest Posted: Wed Oct 27, 2004 7:34 pm    Post subject: Re: typeid operator Andrew Koenig wrote: Quote: "Marco Jez" <nessuno (AT) nessuno (DOT) com> wrote in message news:FORfd.214470$35.10108467 (AT) news4 (DOT) tin.it... I would like to use the reference returned by typeid as key in a std::map. Is it safe to assume that typeid(T) (where T is a type name) will always return the same reference to the same type_info structure for a given T? I don't see why you should be able to make that assumption. I think the Standard in 5.2.8 says that the lvalue is returned and that the lifetime of the object referred to by the lvalue is entire program. So, why should we be able to make that assumption? Or did you forget the 'not' as in "I don't see why you should _not_ be able..."? The only doubt I have is that the OP's map is made to have the reference as the key type. Is it possible? "Key" is required to be assignable. Are references assignable? I kind of think they are fine, but are they? Quote: The type_info::before member is intended to be used for defining a comparison function that you can use for making map keys out of typeids. If you use it, two distinct type_info objects that represent the same type should be treated as equivalent keys. V Back to top Jonathan Turkanis Guest Posted: Wed Oct 27, 2004 7:55 pm    Post subject: Re: typeid operator "Victor Bazarov" <v.Abazarov (AT) comAcast (DOT) net> wrote Quote: Andrew Koenig wrote: "Marco Jez" <nessuno (AT) nessuno (DOT) com> wrote in message news:FORfd.214470$35.10108467 (AT) news4 (DOT) tin.it... I would like to use the reference returned by typeid as key in a std::map. Is it safe to assume that typeid(T) (where T is a type name) will always return the same reference to the same type_info structure for a given T? I don't see why you should be able to make that assumption. I think the Standard in 5.2.8 says that the lvalue is returned and that the lifetime of the object referred to by the lvalue is entire program. So, why should we be able to make that assumption? Or did you forget the 'not' as in "I don't see why you should _not_ be able..."? Wouldn't it be legal -- but insane -- for an implementation to have several type_info objects for a given type, each of which has lifetime equal to the entire program, and for typeid to to select one at random each time typeid is invoked? Perhaps support for dynamic libraries might sometimes lead to the existence of two typeid objects for the same type. Jonathan Back to top Marco Jez Guest Posted: Wed Oct 27, 2004 8:22 pm    Post subject: Re: typeid operator "Andrew Koenig" <ark (AT) acm (DOT) org> ha scritto nel messaggio news:8uSfd.787921$Gx4.579909 (AT) bgtnsc04-news (DOT) ops.worldnet.att.net... Quote: I don't see why you should be able to make that assumption. That's why I've asked... Since it seemed to be true in my implementation I was wondering whether the Standard guarantees that or not. Anyway I didn't notice the before() member, my fault. Quote: The type_info::before member is intended to be used for defining a comparison function that you can use for making map keys out of typeids. If you use it, two distinct type_info objects that represent the same type should be treated as equivalent keys. So, would this map be guaranteed to work as expected? struct Type_info_cmp { bool operator()(const std::type_info *t1, const std::type_info *t2) const { return t1->before(*t2) != 0; } }; typedef std::map<const std::type_info*, My_struct, Type_info_cmp> Type_map; Cheers, Marco Back to top Marco Jez Guest Posted: Wed Oct 27, 2004 8:27 pm    Post subject: Re: typeid operator Quote: The only doubt I have is that the OP's map is made to have the reference as the key type. Is it possible? "Key" is required to be assignable. Are references assignable? I kind of think they are fine, but are they? I was actually thinking of storing a pointer to the type_info structure, not a reference (I've corrected it in my reply to Andrew). Sorry for the confusion. Marco Back to top Andrew Koenig Guest Posted: Wed Oct 27, 2004 8:34 pm    Post subject: Re: typeid operator "Victor Bazarov" <v.Abazarov (AT) comAcast (DOT) net> wrote Quote: Andrew Koenig wrote: I would like to use the reference returned by typeid as key in a std::map. Is it safe to assume that typeid(T) (where T is a type name) will always return the same reference to the same type_info structure for a given T? I don't see why you should be able to make that assumption. I think the Standard in 5.2.8 says that the lvalue is returned and that the lifetime of the object referred to by the lvalue is entire program. So, why should we be able to make that assumption? Or did you forget the 'not' as in "I don't see why you should _not_ be able..."? Because I don't see any place in 5.2.8 that prohibits an implementation from having several distinct objects that represent a given type and returning a reference to one or another of those objects at whim. As a more realistic example, consider a class defined in a header file that is included in several translation units. Where does it say that the implementation can't create a separate type_info object in each translation unit? Back to top Andrew Koenig Guest Posted: Wed Oct 27, 2004 8:36 pm    Post subject: Re: typeid operator "Marco Jez" <guess (AT) none (DOT) it> wrote Quote: "Andrew Koenig" <ark (AT) acm (DOT) org> ha scritto nel messaggio The type_info::before member is intended to be used for defining a comparison function that you can use for making map keys out of typeids. If you use it, two distinct type_info objects that represent the same type should be treated as equivalent keys. So, would this map be guaranteed to work as expected? struct Type_info_cmp { bool operator()(const std::type_info *t1, const std::type_info *t2) const { return t1->before(*t2) != 0; } }; typedef std::map Type_map; That's the idea. [The type_info::before member is in the standard because I proposed it, so I am confident in knowing its purpose ] Back to top Andrey Tarasevich Guest Posted: Wed Oct 27, 2004 8:49 pm    Post subject: Re: typeid operator Marco Jez wrote: Quote: ... I would like to use the reference returned by typeid as key in a std::map. Is it safe to assume that typeid(T) (where T is a type name) will always return the same reference to the same type_info structure for a given T? My map would look like this: typedef std::map<const std::type_info &, ......> Type_map; ... You cannot use any reference type as a key in an 'std::map' because reference types are not Assignable, which happens to be a requirement for 'std::map's key type. Use pointer to 'std::type_info' instead. Since the C++ standard doesn't explicitly guarantee that 'typeid' always return a reference to the same 'type_info' object for the same argument type, you'll have to supply the map with your own comparison predicate, which will perform "intellegent" comparison of 'type_info' pointers by comparing the pointed objects using 'std::type_info::before' method. -- Best regards, Andrey Tarasevich Back to top Andrey Tarasevich Guest Posted: Wed Oct 27, 2004 8:53 pm    Post subject: Re: typeid operator Victor Bazarov wrote: Quote: ... The only doubt I have is that the OP's map is made to have the reference as the key type. Is it possible? "Key" is required to be assignable. Are references assignable? I kind of think they are fine, but are they? ... They are not Assignable of course, by which I mean that any attempt to syntactically "assign" them will actually be applied to referenced objects. Firstly, that's definitely not what the OP wants. Secondly, it won't work because the referenced objects are const-qualified anyway. Thirdly, it won't work because 'std::type_info' objects are not Assignable themselves. -- Best regards, Andrey Tarasevich Back to top Patrick Kowalzick Guest Posted: Wed Oct 27, 2004 9:17 pm    Post subject: Re: typeid operator Hi Marco, this is covered as well in the Loki Library. There is a class storing the pointer to a type_info and comparing via the before function. A short version looks like this: class CTypeInfo { public: CTypeInfo(const std::type_info& ti) : pInfo_(&ti) // non-explicit { assert(pInfo_); } // Access for the wrapped std::type_info const std::type_info& Get() const { assert(pInfo_); return *pInfo_; } // Compatibility functions bool before(const CTypeInfo & rhs) const { assert(pInfo_); return pInfo_->before(*rhs.pInfo_) != 0; } private: const std::type_info* pInfo_; }; // Implementation inline bool operator<(const CTypeInfo& lhs, const CTypeInfo& rhs) { return lhs.before(rhs); } You can use this class directly in a map then. Regards, Patrick Back to top Andrey Tarasevich Guest Posted: Wed Oct 27, 2004 9:37 pm    Post subject: Re: typeid operator Patrick Kowalzick wrote: Quote: class CTypeInfo { ... // Compatibility functions bool before(const CTypeInfo & rhs) const { assert(pInfo_); assert(pInfo_ && rhs.pInfo_) would better fit the general style of using assertions in this code. Quote: return pInfo_->before(*rhs.pInfo_) != 0; } ... -- Best regards, Andrey Tarasevich Back to top Patrick Kowalzick Guest Posted: Wed Oct 27, 2004 10:20 pm    Post subject: Re: typeid operator Quote: would better fit the general style of using assertions in this code. return pInfo_->before(*rhs.pInfo_) != 0; } true :) Thx, Patrick Back to top Tom Widmer Guest Posted: Thu Oct 28, 2004 10:21 am    Post subject: Re: typeid operator On Wed, 27 Oct 2004 13:55:18 -0600, "Jonathan Turkanis" <technews (AT) kangaroologic (DOT) com> wrote: Quote: "Victor Bazarov" <v.Abazarov (AT) comAcast (DOT) net> wrote in message news:0LSfd.7263$Ae.1053 (AT) newsread1 (DOT) dllstx09.us.to.verio.net... Andrew Koenig wrote: "Marco Jez" <nessuno (AT) nessuno (DOT) com> wrote in message news:FORfd.214470$35.10108467 (AT) news4 (DOT) tin.it... I would like to use the reference returned by typeid as key in a std::map. Is it safe to assume that typeid(T) (where T is a type name) will always return the same reference to the same type_info structure for a given T? I don't see why you should be able to make that assumption. I think the Standard in 5.2.8 says that the lvalue is returned and that the lifetime of the object referred to by the lvalue is entire program. So, why should we be able to make that assumption? Or did you forget the 'not' as in "I don't see why you should _not_ be able..."? Wouldn't it be legal -- but insane -- for an implementation to have several type_info objects for a given type, each of which has lifetime equal to the entire program, and for typeid to to select one at random each time typeid is invoked? Perhaps support for dynamic libraries might sometimes lead to the existence of two typeid objects for the same type. It does under Windows. Call typeid on the same (non-exported) type in a DLL and a exe, and the object returned will be different. This is also why MS's dynamic_cast implementation is so slow - it performs string comparisons to work out whether two types are the same, whereas under Unix I think address comparisons are usually sufficient. Tom Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ language (comp.lang.c++) All times are GMT Goto page 1, 2  Next Page 1 of 2   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.