C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  NULL==0?          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Quantum Guest Posted: Tue Oct 10, 2006 9:10 am    Post subject: NULL==0? Hi, Are these equivalent: char text[]; if(text==NULL){} if(text==0){} Thank you, Q Back to top Quantum Guest Posted: Tue Oct 10, 2006 9:10 am    Post subject: Re: NULL==0? Nick Keighley wrote: Quote: Quantum wrote: Are these equivalent: char text[]; if(text==NULL){} if(text==0){} yes, but, since text is an array it can never be NULL... Quantum wrote: Are these equivalent: char text[]; if(text==NULL){} if(text==0){} yes, but, since text is an array it can never be NULL... Good point! I'm passing text[] into a function, however it is still declared as a pointer. Back to top Nick Keighley Guest Posted: Tue Oct 10, 2006 9:10 am    Post subject: Re: NULL==0? Quantum wrote: Quote: Are these equivalent: char text[]; if(text==NULL){} if(text==0){} yes, but, since text is an array it can never be NULL... -- Nick Keighley Back to top Quantum Guest Posted: Tue Oct 10, 2006 9:10 am    Post subject: Re: NULL==0? Papastefanos Serafeim wrote: Quote: "Quantum" <noemail (AT) address (DOT) com> wrote in message news:6NHWg.10854$Fx4.6732@newsfe1-gui.ntli.net... Hi, Are these equivalent: char text[]; if(text==NULL){} if(text==0){} Thank you, Q Although it's from the C faq it addresses your question http://c-faq.com/null/index.html Serafeim Ok, thanks. Q Back to top Papastefanos Serafeim Guest Posted: Tue Oct 10, 2006 9:10 am    Post subject: Re: NULL==0? "Quantum" <noemail (AT) address (DOT) com> wrote in message news:6NHWg.10854$Fx4.6732@newsfe1-gui.ntli.net... Quote: Hi, Are these equivalent: char text[]; if(text==NULL){} if(text==0){} Thank you, Q Although it's from the C faq it addresses your question http://c-faq.com/null/index.html Serafeim Back to top Bart Guest Posted: Fri Oct 13, 2006 9:10 am    Post subject: Re: NULL==0? jmcgill wrote: Quote: Strictly speaking they are equivalent, but idiomatically, the use of the NULL macro indicates that you are evaluating a pointer, and not just evaluating some integer. Idiomatically, I like to use if(!ptr) because it looks the same as if it was a boolean, and it makes sense to me: "if not ptr" is like "if there's nothing in ptr" or "if ptr is not pointing to anything". Quote: Comparing to NULL and comparing to an integer value of zero are equivalent in you example, but they are not equivalent everywhere NULL may be used. They are always equivalent. It's guaranteed by the standard. Quote: This is because NULL does not require a typecast in order to be passed as a parameter to a function that expects a pointer, but a plain int would not necessarily satisfy the type requirements of the function prototype. NULL cannot be defined as anything else than zero in C++ so that is incorrect. NULL is the same as plain 0 as far as overloading is concerned. Quote: You might still get away with this, because all your target platforms probably really do have a NULL pointer that is an integer value with "all bits off", and all your target platforms probably really do represent the integer value of zero as a two's complement number with "all bits off." But, this is really just a happy coincidence. What happens on a platform where "zero" is something else? How about on a one's complement system where there are TWO zeros? And what happens on a platform where the NULL pointer, while represented in the high level language as "zero", really is some other value, like the address of some hardware trap with a segment and offset value, different for each program? You're confusing stuff. In a C++ program the constant 0 IS the null pointer. It doesn't matter what the representation of it is in the hardware. If you use 0 in a pointer expression it will have the correct representation. This is NOT the same as memcmp-ing with zero or memset-ing a pointer to zero. Quote: So, this doesn't happen on x86, Sparc, ARM, MIPS, PPC, 68K, or any other machine you are likely to be using. And maybe it's irrelevant to ask if you'd bet your life on it remaining so. But it is nonetheless appropriate to practice the good habit of using the NULL macro for the null pointer. Talk for yourself. IMO, it is not a good habit to use any unnecessary macro, which NULL is. NULL is a historical accident like several other things in the language. Regards, Bart. Back to top Clark S. Cox III Guest Posted: Sun Oct 15, 2006 6:30 am    Post subject: Re: NULL==0? Old Wolf wrote: Quote: Frederick Gotham wrote: The macro, NULL, must be a compile-time integer constant which evaluates to zero. No more. No less. Not true. It could be anything as long as it can be converted to a pointer, resulting in a null pointer. For example, GCC defines NULL as __nullptr, which is a value that meets this requirement. This definition does not break any standards conformance. __nullptr in GCC is still a constant integer expression that evaluates to zero. Were it not, it could not be a null pointer constant. -- Clark S. 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