C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Why do we have void*?          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message Tomás Guest Posted: Mon May 15, 2006 9:21 pm    Post subject: Why do we have void*? This is probably more of a question for a C newsgroup, but nonetheless C++ is now a language in and of itself -- so everything goes. Every object is made up of bytes, and a "char*" can store the address of a byte. So why do we have "void*" in the language? Quote: From what I can see, "char*" is perfectly sufficient for storing any address. Plus, it has the added functionality of pointer arithmetic. -Tomás --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Gene Bushuyev Guest Posted: Tue May 16, 2006 5:21 am    Post subject: Re: Why do we have void*? ""Tomás"" <NULL (AT) NULL (DOT) NULL> wrote in message news:ue4ag.9209$j7.305609 (AT) news (DOT) indigo.ie... This is probably more of a question for a C newsgroup, but nonetheless C++ is now a language in and of itself -- so everything goes. Every object is made up of bytes, and a "char*" can store the address of a byte. So why do we have "void*" in the language? Quote: From what I can see, "char*" is perfectly sufficient for storing any address. Plus, it has the added functionality of pointer arithmetic. void* is the only pointer type that you can cast the pointer to any other object type to and from and get exactly what you had before. -- Gene Bushuyev (www.gbresearch.com) ---------------------------------------------------------------- There is no greatness where there is no simplicity, goodness and truth. ~ Leo Tolstoy --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Tomás Guest Posted: Tue May 16, 2006 3:24 pm    Post subject: Re: Why do we have void*? Gene Bushuyev posted: Quote: void* is the only pointer type that you can cast the pointer to any other object type to and from and get exactly what you had before. A "char*" should have that exact ability, as it can address a single byte -- the smallest chunk of memory in C++. My argument is that a char* has all the functionality of a void*, so what can a void* do that a char* can't? char* even has the added bonus of pointer arithmetic. -Tomás --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Kapil Guest Posted: Tue May 16, 2006 3:24 pm    Post subject: Re: Why do we have void*? I think this is really to stop you from doing pointer airthmetic if you dont know what type of object is. Isn't it sound in regard of Software Engineering Principles --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Guest Posted: Tue May 16, 2006 4:21 pm    Post subject: Re: Why do we have void*? "Tomás" wrote: Quote: This is probably more of a question for a C newsgroup, but nonetheless C++ is now a language in and of itself -- so everything goes. Every object is made up of bytes, and a "char*" can store the address of a byte. So why do we have "void*" in the language? From what I can see, "char*" is perfectly sufficient for storing any address. Plus, it has the added functionality of pointer arithmetic. The purpose of void* is type safety. It's the things you can't do with void* that you can do with char* which make void* so valuable. You can't dereference it, or even do address arithmetic on it, without first converting to some other type, and it won't implicity convert to other types. When code uses void*, it's explicitly saying "I don't know what type of object this pointer is pointing at", which is a very important thing to keep track of, when it's true. This is particularly important in C++, where you have function overloading; you should be able to distinguish the overload which is intended to handle pointers to unknown types from the overload for a pointer to char, and you couldn't do that if char* was used for both purposes. --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Tomás Guest Posted: Wed May 17, 2006 6:21 am    Post subject: Re: Why do we have void*? Kuyper posted: Quote: The purpose of void* is type safety. It's the things you can't do with void* that you can do with char* which make void* so valuable. You can't dereference it, or even do address arithmetic on it, without first converting to some other type, and it won't implicity convert to other types. When code uses void*, it's explicitly saying "I don't know what type of object this pointer is pointing at", which is a very important thing to keep track of, when it's true. This is particularly important in C++, where you have function overloading; you should be able to distinguish the overload which is intended to handle pointers to unknown types from the overload for a pointer to char, and you couldn't do that if char* was used for both purposes. I see, nice explanation, thanks. Am I right though in thinking that a "char*" can store any address reliably? (I'm pretty sure of this, but just when you're sure about something, someone points something out which you missed the first time round). -Tomás --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Zara Guest Posted: Wed May 17, 2006 6:21 am    Post subject: Re: Why do we have void*? On Tue, 16 May 2006 14:53:51 GMT, NULL (AT) NULL (DOT) NULL ("Tomás") wrote: Quote: Gene Bushuyev posted: void* is the only pointer type that you can cast the pointer to any other object type to and from and get exactly what you had before. A "char*" should have that exact ability, as it can address a single byte -- the smallest chunk of memory in C++. My argument is that a char* has all the functionality of a void*, so what can a void* do that a char* can't? char* even has the added bonus of pointer arithmetic. -Tomás The greatest adavantage of "void *" over "char *" is that it can never be (mis)used by itself, you need to reinterpret_cast it. char * may be inadvertently used within a stream operation. If you were trying to see the numerical value of the pointer, you might get lots of grabage and (probably) some kind of memory access error. Zara --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top David R Tribble Guest Posted: Wed May 17, 2006 6:21 am    Post subject: Re: Why do we have void*? Tomás wrote: Quote: This is probably more of a question for a C newsgroup, but nonetheless C++ is now a language in and of itself -- so everything goes. Every object is made up of bytes, and a "char*" can store the address of a byte. So why do we have "void*" in the language? From what I can see, "char*" is perfectly sufficient for storing any address. Plus, it has the added functionality of pointer arithmetic. Kuyper wrote: Quote: The purpose of void* is type safety. It's the things you can't do with void* that you can do with char* which make void* so valuable. You can't dereference it, or even do address arithmetic on it, without first converting to some other type, and it won't implicity convert to other types. When code uses void*, it's explicitly saying "I don't know what type of object this pointer is pointing at", which is a very important thing to keep track of, when it's true. This is particularly important in C++, where you have function overloading; you should be able to distinguish the overload which is intended to handle pointers to unknown types from the overload for a pointer to char, and you couldn't do that if char* was used for both purposes. Yep. Consider 'void' equivalent to 'untyped' or 'notype', i.e., 'void*' is 'pointer to untyped memory byte(s)'. It would probably have been less confusing if they'd chosen a different word than 'void', but that's water under the bridge now. It's primarily used for passing pointers to objects when you don't care what the actual type of the objects are, when you want to manipulate the object as just collections of bytes (which is what the malloc() and free() functions do). It's also useful when you want to stash a pointer to an object provided by some code that is outside your own code, so that you can return it to that outside code later on; you don't care what the object actually is, you just want to save its address for later use. And it's useful for providing an "opaque" pointer to an object of some type that you want to keep hidden from client code. Your code allocates the object and passes its pointer back as an untyped pointer to the client, and the client passes it back to your code without knowing what it really points to, and you convert it to the proper pointer type within your code. All of these things could be done using 'char*', but then you don't get the benefit of type safety, because the compiler assumes (rightly) that it's a pointer to a character object, when in fact it probably is not. -drt --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top kanze Guest Posted: Wed May 17, 2006 5:21 pm    Post subject: Re: Why do we have void*? "Tomás" wrote: Quote: Am I right though in thinking that a "char*" can store any address reliably? Yes. Void was introduced relatively late in C, for reasons of type safety. Before that, char* was what was used, and it was felt necessary to support existing code. The other point, of course, is that unsigned char (and in C++, char as well) has been blessed as a means of accessing raw memory. Since you look at any object as an array of char, char* pretty much has to be able to address any object. -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Greg Herlihy Guest Posted: Wed May 17, 2006 7:21 pm    Post subject: Re: Why do we have void*? David R Tribble wrote: Quote: Tomás wrote: This is probably more of a question for a C newsgroup, but nonetheless C++ is now a language in and of itself -- so everything goes. Every object is made up of bytes, and a "char*" can store the address of a byte. So why do we have "void*" in the language? From what I can see, "char*" is perfectly sufficient for storing any address. Plus, it has the added functionality of pointer arithmetic. Kuyper wrote: The purpose of void* is type safety. It's the things you can't do with void* that you can do with char* which make void* so valuable. You can't dereference it, or even do address arithmetic on it, without first converting to some other type, and it won't implicity convert to other types. When code uses void*, it's explicitly saying "I don't know what type of object this pointer is pointing at", which is a very important thing to keep track of, when it's true. This is particularly important in C++, where you have function overloading; you should be able to distinguish the overload which is intended to handle pointers to unknown types from the overload for a pointer to char, and you couldn't do that if char* was used for both purposes. Yep. Consider 'void' equivalent to 'untyped' or 'notype', i.e., 'void*' is 'pointer to untyped memory byte(s)'. It would probably have been less confusing if they'd chosen a different word than 'void', but that's water under the bridge now. ... And it's useful for providing an "opaque" pointer to an object of some type that you want to keep hidden from client code. Your code allocates the object and passes its pointer back as an untyped pointer to the client, and the client passes it back to your code without knowing what it really points to, and you convert it to the proper pointer type within your code. An opaque pointer should be declared as a pointer to a forwardly-declared, but never defined type - not as a pointer to void. Otherwise the client will be free to pass any kind of pointer to a routine that requires a pointer to the opaque type. Only in the case that a function is actually willing to accept a pointer to any type, should it declare a void * parameter. Greg --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Stephan Bergmann Guest Posted: Thu May 18, 2006 6:21 am    Post subject: Re: Why do we have void*? David R Tribble wrote: [...] Quote: And it's useful for providing an "opaque" pointer to an object of some type that you want to keep hidden from client code. Your code allocates the object and passes its pointer back as an untyped pointer to the client, and the client passes it back to your code without knowing what it really points to, and you convert it to the proper pointer type within your code. [...] -drt In such a case, it is probably even better (i.e., more type safe) to leave the object type incomplete for the client code, and use a pointer to that incomplete object type, instead of a void pointer, at the interface. -Stephan --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top David R Tribble Guest Posted: Fri May 19, 2006 12:21 am    Post subject: Re: Why do we have void*? David R Tribble wrote: Quote: And [void*] is useful for providing an "opaque" pointer to an object of some type that you want to keep hidden from client code. Your code allocates the object and passes its pointer back as an untyped pointer to the client, and the client passes it back to your code without knowing what it really points to, and you convert it to the proper pointer type within your code. Greg Herlihy wrote: Quote: An opaque pointer should be declared as a pointer to a forwardly-declared, but never defined type - not as a pointer to void. Otherwise the client will be free to pass any kind of pointer to a routine that requires a pointer to the opaque type. Only in the case that a function is actually willing to accept a pointer to any type, should it declare a void * parameter. Yep, such as when your code has to store an opaque pointer to an object managed by some other third-party code. That third-party code might vary quite a lot (if you link to different libraries on different platforms, for instance), so you're forced to store the pointer in its most generic form, which is a void*. -drt --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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