C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  keeping a pointer to typeinfo?          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message John Torjo Guest Posted: Fri Jul 16, 2004 11:17 am    Post subject: keeping a pointer to typeinfo? Hi all, Heres's what the standard says: [expr.typeid] 5.2.8 Type identification The result of a typeid expression is an lvalue of static type const std::type_info (18.5.1) and dynamic type const std::type_info or const name where name is an implementation defined class derived from std::type_info which preserves the behavior described in 18.5.1. The lifetimeof the object referred to by the lvalue extends to the end of the program. Whether or not the destructor is called for the type_info object at the end of the program is unspecified. Quick question: If I read this correctly, I can hold a pointer to dereferencing a typeid(), and that pointer will last throughout the program. Am I correct? Example: some_class * p = ...; // this will last throughout the program const std::type_info * info = *typeid(*p); Thanks. Best, John John Torjo Freelancer -- [email]john (AT) torjo (DOT) com[/email] Contributing editor, C/C++ Users Journal -- "Win32 GUI Generics" -- generics & GUI do mix, after all -- http://www.torjo.com/win32gui/ Professional Logging Solution for FREE -- http://www.torjo.com/code/logging.zip (logging - C++) -- http://www.torjo.com/logview/ (viewing/filtering - Win32) -- http://www.torjo.com/logbreak/ (debugging - Win32) (source code available) [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top tom_usenet Guest Posted: Mon Jul 19, 2004 5:23 pm    Post subject: Re: keeping a pointer to typeinfo? On 16 Jul 2004 07:17:27 -0400, [email]jtorjo (AT) yahoo (DOT) com[/email] (John Torjo) wrote: Quote: Hi all, Heres's what the standard says: [expr.typeid] 5.2.8 Type identification The result of a typeid expression is an lvalue of static type const std::type_info (18.5.1) and dynamic type const std::type_info or const name where name is an implementation defined class derived from std::type_info which preserves the behavior described in 18.5.1. The lifetimeof the object referred to by the lvalue extends to the end of the program. Whether or not the destructor is called for the type_info object at the end of the program is unspecified. Quick question: If I read this correctly, I can hold a pointer to dereferencing a typeid(), and that pointer will last throughout the program. Am I correct? Example: some_class * p = ...; // this will last throughout the program const std::type_info * info = *typeid(*p); You want: const std::type_info* info = &typeid(*p); That pointer is now valid for the duration of the program. Tom [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top John Torjo Guest Posted: Tue Jul 20, 2004 8:13 am    Post subject: Re: keeping a pointer to typeinfo? tom_usenet <tom_usenet (AT) hotmail (DOT) com> wrote Quote: On 16 Jul 2004 07:17:27 -0400, [email]jtorjo (AT) yahoo (DOT) com[/email] (John Torjo) wrote: Hi all, Heres's what the standard says: [expr.typeid] 5.2.8 Type identification The result of a typeid expression is an lvalue of static type const std::type_info (18.5.1) and dynamic type const std::type_info or const name where name is an implementation defined class derived from std::type_info which preserves the behavior described in 18.5.1. The lifetimeof the object referred to by the lvalue extends to the end of the program. Whether or not the destructor is called for the type_info object at the end of the program is unspecified. Quick question: If I read this correctly, I can hold a pointer to dereferencing a typeid(), and that pointer will last throughout the program. Am I correct? Example: some_class * p = ...; // this will last throughout the program const std::type_info * info = *typeid(*p); You want: const std::type_info* info = &typeid(*p); That pointer is now valid for the duration of the program. Of course, that's what I meant these typos ;) (basically, only after hitting the "Post message - No preview" I realized the mistke) Thanks. Best, John John Torjo Freelancer -- [email]john (AT) torjo (DOT) com[/email] Contributing editor, C/C++ Users Journal -- "Win32 GUI Generics" -- generics & GUI do mix, after all -- http://www.torjo.com/win32gui/ Professional Logging Solution for FREE -- http://www.torjo.com/code/logging.zip (logging - C++) -- http://www.torjo.com/logview/ (viewing/filtering - Win32) -- http://www.torjo.com/logbreak/ (debugging - Win32) (source code available) [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top tom_usenet Guest Posted: Tue Jul 20, 2004 6:20 pm    Post subject: Re: keeping a pointer to typeinfo? On 20 Jul 2004 04:13:55 -0400, [email]jtorjo (AT) yahoo (DOT) com[/email] (John Torjo) wrote: Quote: tom_usenet <tom_usenet (AT) hotmail (DOT) com> wrote On 16 Jul 2004 07:17:27 -0400, [email]jtorjo (AT) yahoo (DOT) com[/email] (John Torjo) wrote: Hi all, Heres's what the standard says: [expr.typeid] 5.2.8 Type identification The result of a typeid expression is an lvalue of static type const std::type_info (18.5.1) and dynamic type const std::type_info or const name where name is an implementation defined class derived from std::type_info which preserves the behavior described in 18.5.1. The lifetimeof the object referred to by the lvalue extends to the end of the program. Whether or not the destructor is called for the type_info object at the end of the program is unspecified. Quick question: If I read this correctly, I can hold a pointer to dereferencing a typeid(), and that pointer will last throughout the program. Am I correct? Example: some_class * p = ...; // this will last throughout the program const std::type_info * info = *typeid(*p); You want: const std::type_info* info = &typeid(*p); That pointer is now valid for the duration of the program. Of course, that's what I meant these typos One thing I should have mentioned is that &typeid(T) isn't guaranteed always to return the same pointer; in particular, it does on Unix with it's shared library system, it might not on Windows with it's independent DLL system. Tom [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top David Abrahams Guest Posted: Tue Jul 20, 2004 9:40 pm    Post subject: Re: keeping a pointer to typeinfo? tom_usenet <tom_usenet (AT) hotmail (DOT) com> writes: Quote: On 20 Jul 2004 04:13:55 -0400, [email]jtorjo (AT) yahoo (DOT) com[/email] (John Torjo) wrote: tom_usenet <tom_usenet (AT) hotmail (DOT) com> wrote On 16 Jul 2004 07:17:27 -0400, [email]jtorjo (AT) yahoo (DOT) com[/email] (John Torjo) wrote: Hi all, Heres's what the standard says: [expr.typeid] 5.2.8 Type identification The result of a typeid expression is an lvalue of static type const std::type_info (18.5.1) and dynamic type const std::type_info or const name where name is an implementation defined class derived from std::type_info which preserves the behavior described in 18.5.1. The lifetimeof the object referred to by the lvalue extends to the end of the program. Whether or not the destructor is called for the type_info object at the end of the program is unspecified. Quick question: If I read this correctly, I can hold a pointer to dereferencing a typeid(), and that pointer will last throughout the program. Am I correct? Example: some_class * p = ...; // this will last throughout the program const std::type_info * info = *typeid(*p); You want: const std::type_info* info = &typeid(*p); That pointer is now valid for the duration of the program. Of course, that's what I meant these typos ;) One thing I should have mentioned is that &typeid(T) isn't guaranteed always to return the same pointer; in particular, it does on Unix with it's shared library system, it might not on Windows with it's independent DLL system. It's easy to make various Unices including G++/Linux display a similar behavior if you use dlopen(). In fact, two typeid(T)s from different shared libraries can compare unequal for the same T. :vP -- Dave Abrahams Boost Consulting http://www.boost-consulting.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top John Torjo Guest Posted: Wed Jul 21, 2004 12:04 pm    Post subject: Re: keeping a pointer to typeinfo? Quote: Of course, that's what I meant these typos ;) One thing I should have mentioned is that &typeid(T) isn't guaranteed always to return the same pointer; in particular, it does on Unix with That should not count. What I want is, by having two typeinfo pointers, to be able to say: *p1 == *p2 Quote: it's shared library system, it might not on Windows with it's independent DLL system. It's easy to make various Unices including G++/Linux display a similar behavior if you use dlopen(). In fact, two typeid(T)s from different shared libraries can compare unequal for the same T. :( Luckly, this should not be my case. Specifically, I only want it for Win32. What I want to achieve is something similar to this: struct employee { ... }; struct department { ... }; employee e; department d; corresp c; c.add_var(e); // template function c.add_var(d); // here, I want to "bind" this to the variable "d" c.add_corresp(&department::department_id, ID_dep); // same here c.add_corresp(&department::department_name, ID_dep_name); // here, bind this to variable "e" c.add_corresp(&employee::employee_name, ID_name); Best, John John Torjo Freelancer -- [email]john (AT) torjo (DOT) com[/email] Contributing editor, C/C++ Users Journal -- "Win32 GUI Generics" -- generics & GUI do mix, after all -- http://www.torjo.com/win32gui/ Professional Logging Solution for FREE -- http://www.torjo.com/code/logging.zip (logging - C++) -- http://www.torjo.com/logview/ (viewing/filtering - Win32) -- http://www.torjo.com/logbreak/ (debugging - Win32) (source code available) [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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