C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  RTTI and DLLs          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Tobias Wagner Guest Posted: Thu Jun 09, 2005 9:39 am    Post subject: RTTI and DLLs I have a DLL which exports some functions, say class A; class B; A* create_A(); B* create_B(); void do_something(A* a, B* b); This DLL is called from other languages (Delphi). In do_something I'd like to check if 'a' is really an A* and 'b' is really a B*. I tried: void do_something(A* a, B* b) { if (not dynamic_cast <A*> (a)) std::cerr << "Error: wrong typen"; But this does not work - obviously at least from the compiler point of view... How can I have this kind of "type safty" between different languages? Do I need to add a typeid variable to all classes? Cheers, Tobi [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ron Natalie Guest Posted: Thu Jun 09, 2005 12:36 pm    Post subject: Re: RTTI and DLLs Tobias Wagner wrote: Quote: I have a DLL which exports some functions, say class A; class B; A* create_A(); B* create_B(); void do_something(A* a, B* b); This DLL is called from other languages (Delphi). In do_something I'd like to check if 'a' is really an A* and 'b' is really a B*. I tried: void do_something(A* a, B* b) { if (not dynamic_cast <A*> (a)) std::cerr << "Error: wrong typen"; Define "doesn't work?" If you want to check exact type, use typeid. dyanmic_cast just checks to see if it's dynamically convertable. The above experssion is a nop. All it does is test to see if a is a NULL pointer. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top msalters Guest Posted: Fri Jun 10, 2005 10:16 am    Post subject: Re: RTTI and DLLs Tobias Wagner schreef: Quote: I have a DLL which exports some functions, say class A; class B; A* create_A(); B* create_B(); void do_something(A* a, B* b); This DLL is called from other languages (Delphi). In do_something I'd like to check if 'a' is really an A* and 'b' is really a B*. What else can it be? If anotherlanguage is forcing an X through, there's very little you can do about it anyway. Quote: I tried: void do_something(A* a, B* b) { if (not dynamic_cast <A*> (a)) std::cerr << "Error: wrong typen"; But this does not work - obviously at least from the compiler point of view... WIthout seeing your class A, I'd have to guess A doesn't have a virtual function. dynamic_cast only works on conversions in an inheritance relation. Every A object is considered to inherit from A, so the cast is effectively an identity conversion and is likely to be optimized out. It certainly is unlikely to catch an unrelated X. Quote: How can I have this kind of "type safty" between different languages? Not. There's no way you can detect all kind of errenous input. What if the other language doesn't pass a pointer at all, but an int? Not an int*, but really an int? That's often equivalent to a reinterpret_cast typeid you invent will try to read some bits from the A*, but reading bits from address reinterpret_cast<A*>( 5 ) will simply not work. HTH, Michiel Salters [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Tobias Wagner Guest Posted: Fri Jun 10, 2005 10:22 am    Post subject: Re: RTTI and DLLs Ron Natalie wrote: Quote: I tried: void do_something(A* a, B* b) { if (not dynamic_cast <A*> (a)) std::cerr << "Error: wrong typen"; Define "doesn't work?" It's never an error. Even if a pointer to a random adress is given. It is casted to an A* and dynamic_cast returns != 0, although *a is garbage. Example in Delphi: a, b : Pointer begin a := create_A(); b := create_B(); do_something (b, a); // a and b are changed and the DLL should give an error! Quote: If you want to check exact type, use typeid. I want to check, if at the given adress is an object of type A. Quote: dyanmic_cast just checks to see if it's dynamically convertable. The above experssion is a nop. All it does is test to see if a is a NULL pointer. I know. dynamic_cast is NULL iff the argument is not of the given type. I thought that with RTTI enabled, each object has an additional field with a unique type-identifier. With the dynamic_cast statement I want to check if this identifier has the same identifier as a given type. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top enrique Guest Posted: Fri Jun 10, 2005 10:24 am    Post subject: Re: RTTI and DLLs Your question really sounds like a Windows programming question. Your Delphi app needs to interact with your (C++-generated) DLL via COM. Check out usenet for related groups that discuss this specifically. [ 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: Fri Jun 10, 2005 3:46 pm    Post subject: Re: RTTI and DLLs In article <d89l24$3be$1 (AT) news2 (DOT) rz.uni-karlsruhe.de>, Tobias Wagner <none (AT) rz (DOT) uni-karlsruhe.de> writes Quote: I know. dynamic_cast is NULL iff the argument is not of the given type. Not really, dynamic_cast<T*> (a_ptr) returns a null pointer iff a_ptr cannot be converted to point to an object of type T. Note that the actual object pointed to by a_ptr may be a base sub-object of a complete object that contains another base sub-object of type T. Or put in more normal terms, dynamic_cast<> is capable of up-casting, down-casting _and_ cross-casting. Quote: I thought that with RTTI enabled, each object has an additional field with a unique type-identifier. No, every type with a virtual member has a mechanism for dynamically identifying the exact type. Other types are dealt with statically. Quote: With the dynamic_cast statement I want to check if this identifier has the same identifier as a given type. Then do what you want; compare the typeids -- 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. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Fri Jun 10, 2005 4:35 pm    Post subject: Re: RTTI and DLLs In article <1118343446.388820.225030 (AT) o13g2000cwo (DOT) googlegroups.com>, enrique <enrique.pineda (AT) gmail (DOT) com> writes Quote: Your question really sounds like a Windows programming question. Your Delphi app needs to interact with your (C++-generated) DLL via COM. Check out usenet for related groups that discuss this specifically. Whilst the motivation was to do with Windows programming, the question was very clearly about C++ which is why it was accepted. -- 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. First time posters: Do this! ] Back to top Maxim Yegorushkin Guest Posted: Fri Jun 10, 2005 4:36 pm    Post subject: Re: RTTI and DLLs On Thu, 09 Jun 2005 16:36:13 +0400, Ron Natalie <ron (AT) spamcop (DOT) net> wrote: [] Quote: Define "doesn't work?" If you want to check exact type, use typeid. dyanmic_cast just checks to see if it's dynamically convertable. The above experssion is a nop. All it does is test to see if a is a NULL pointer. In addition, to be able to use dynamic_cast, both classes in question must have a virtual function. -- Maxim Yegorushkin [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Swampmonster Guest Posted: Sat Jun 11, 2005 10:42 am    Post subject: Re: RTTI and DLLs Quote: I want to check, if at the given adress is an object of type A. You can't with C++. Quote: dyanmic_cast just checks to see if it's dynamically convertable. The above experssion is a nop. All it does is test to see if a is a NULL pointer. I know. dynamic_cast is NULL iff the argument is not of the given type. I thought that with RTTI enabled, each object has an additional field with a unique type-identifier. With the dynamic_cast statement I want to check if this identifier has the same identifier as a given type. See above and the other replies to your question - you can't do that with C++. For all the RTTI stuff to work in C++ you need to have a valid pointer/type pair to begin with. Then you can test for the concrete class - i.e. when you have a class hirarchy like in MFC you can test if a (valid!) CWnd* points to a concrete CButton. Finally, when you pass a CWnd* to dynamic_cast and cast it to CWnd* again, the compiler assumes that the CWnd* you pass in is valid, i.e. points to a CWnd (or derived) object, and just drops the cast - i.e. you will get the exact same value that you passed in, even if it's garbage. One solution to your problem might be to store the addresses of all objects allocated by your DLL inside some container - possibly along with some form of type-id (either a std::string initialized by type_info::raw_name() or some other home-brewn id). Then you can (in each of your DLL's functions) check if the object is known to your DLL (i.e. look it up in that container which would preferrably be a (hash)set or (hash)map) and check if the type stored in the container is what your function expects it to be. Or just use the handle idiom - but that might be overkill in case you don't need to hide the address of your objects. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Paul DeRocco Guest Posted: Sat Jun 11, 2005 10:49 am    Post subject: Re: RTTI and DLLs Quote: "Tobias Wagner" <none (AT) rz (DOT) uni-karlsruhe.de> wrote I have a DLL which exports some functions, say class A; class B; A* create_A(); B* create_B(); void do_something(A* a, B* b); This DLL is called from other languages (Delphi). In do_something I'd like to check if 'a' is really an A* and 'b' is really a B*. I tried: void do_something(A* a, B* b) { if (not dynamic_cast <A*> (a)) std::cerr << "Error: wrong typen"; But this does not work - obviously at least from the compiler point of view... How can I have this kind of "type safty" between different languages? Do I need to add a typeid variable to all classes? Both dynamic_cast and typeid are only able to identify the actual derived class, given a base class pointer, and then only for polymorphic classes. So casting an A* to an A* is a no-op. No language mechanism can verify that a typed pointer hasn't been given a spoofed pointer to something invalid, any more than it can find the actual type of an object given a void pointer. About the best thing you can do, if you really need to protect against spoofing, and you're designing the classes, is to put a member containing a magic number at the beginning of all your classes, and check that. -- Ciao, Paul D. DeRocco Paul mailto:pderocco (AT) ix (DOT) netcom.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Tobias Wagner Guest Posted: Sat Jun 11, 2005 6:29 pm    Post subject: Re: RTTI and DLLs Paul DeRocco wrote: Quote: No language mechanism can verify that a typed pointer hasn't been given a spoofed pointer to something invalid, any more than it can find the actual type of an object given a void pointer. About the best thing you can do, if you really need to protect against spoofing, and you're designing the classes, is to put a member containing a magic number at the beginning of all your classes, and check that. That's what I wanted to do: use the typeid as the "magic number". If someone spoofs me and gives an int instead of an A*, then in many cases at the address where C++ expects to find the typeid of A will be something which is different from the typeid of A or one of its descendants. The only way I could think of testing if the typeid (or magic number) is A or a descendant, was using dynamic_cast. Cheers, Tobi [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top James Kanze Guest Posted: Sun Jun 12, 2005 3:29 pm    Post subject: Re: RTTI and DLLs Tobias Wagner wrote: Quote: Ron Natalie wrote: I tried: void do_something(A* a, B* b) { if (not dynamic_cast <A*> (a)) std::cerr << "Error: wrong typen"; Define "doesn't work?" It's never an error. Even if a pointer to a random adress is given. It is casted to an A* and dynamic_cast returns != 0, although *a is garbage. You have an A*. You ask the system if it can be converted to an A*. What should the system say? Even with dynamic_cast, you cannot lie to the system, and expect to get anything correct back. Quote: Example in Delphi: a, b : Pointer begin a := create_A(); b := create_B(); do_something (b, a); // a and b are changed and the DLL should give an error! If you want to check exact type, use typeid. I want to check, if at the given adress is an object of type A. Which is impossible, given the way C++ works. Also unnecessary; the above call would not compile in C++, since pointers are typed according to what they point to. Quote: dyanmic_cast just checks to see if it's dynamically convertable. The above experssion is a nop. All it does is test to see if a is a NULL pointer. I know. dynamic_cast is NULL iff the argument is not of the given type. I thought that with RTTI enabled, each object has an additional field with a unique type-identifier. With the dynamic_cast statement I want to check if this identifier has the same identifier as a given type. First, dynamic_cast only works for polymorphic objects -- objects with at least one virtual function. The typical implementation puts information in the vtbl, which it uses to implement dynamic cast. No vtbl, no dynamic_cast. The compiler accepts the statement if and only if it can prove that the cast must succeed. (Casting a pointer to its own type always succeeds.) But also, if the pointer does not point to an object which has a pointer to the vtbl, the compiler will access random data. Adding one or more virtual functions will make dynamic_cast work. But will still not change your example, since the compiler knows how to convert an A* into an A* statically, and it knows that the tests in dynamic_cast cannot fail. -- James Kanze mailto: [email]james.kanze (AT) free (DOT) fr[/email] Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 pl. Pierre Sémard, 78210 St.-Cyr-l'École, France +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top James Kanze Guest Posted: Sun Jun 12, 2005 3:30 pm    Post subject: Re: RTTI and DLLs Tobias Wagner wrote: Quote: Paul DeRocco wrote: No language mechanism can verify that a typed pointer hasn't been given a spoofed pointer to something invalid, any more than it can find the actual type of an object given a void pointer. About the best thing you can do, if you really need to protect against spoofing, and you're designing the classes, is to put a member containing a magic number at the beginning of all your classes, and check that. That's what I wanted to do: use the typeid as the "magic number". If someone spoofs me and gives an int instead of an A*, then in many cases at the address where C++ expects to find the typeid of A will be something which is different from the typeid of A or one of its descendants. And in many other cases, attempting to read the magic number will cause the code to core dump. In practice, most C++ implementations do put a "magic number" at the start of a class, provided the class has virtual functions; the "magic number" is chosen (at execution time) so that when dereferenced, it will point to a table of pointers to the virtual functions and other type dependant information. But 1) trying to read this magic number may cause a core dump, if the pointer doesn't point to what it is suppsed to, and 2) the compiler only reads it when it actually needs the information behind it -- it knows how to convert an A* into an A* without any special information, and will normally not read it in a dynamic_cast of an A* to an A* (just as it will normally not read it in a dynamic_cast of a Derived* to a Base*). Quote: The only way I could think of testing if the typeid (or magic number) is A or a descendant, was using dynamic_cast. If you don't know whether the pointer is valid, you can't dereference it, which limits somewhat your options. About the only thing I can think of would be to enrol all of the this pointers in a set in the constructors, and deenrol them in the destructors. Then, check if the pointer you are given is contained in the set. -- James Kanze mailto: [email]james.kanze (AT) free (DOT) fr[/email] Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 pl. Pierre Sémard, 78210 St.-Cyr-l'École, France +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top enrique Guest Posted: Tue Jun 14, 2005 7:38 pm    Post subject: Re: RTTI and DLLs I saw lots of feedback on your question. Did you ever figure this out with a C++ solution? Don't forget to post your results so the rest of us can benefit from your experience. Thanks. epp [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Tobias Wagner Guest Posted: Wed Jun 15, 2005 10:42 pm    Post subject: Re: RTTI and DLLs enrique wrote: Quote: I saw lots of feedback on your question. Yes, but unfortunately the answer in short form is: It doesn't work with standard C++ features. Quote: Did you ever figure this out with a C++ solution? As several authors wrote, the only solution would be to introduce magic numbers in each of my classes. But then I would have to fiddle around with derived classes when checking for types. So I decided to live with the segfaults, if the users of my dynamic library give the wrong types. Thanks to anyone for the helpful comments. Tobi [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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