C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  How does #define operate ?          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Timothy Madden Guest Posted: Mon Sep 27, 2004 2:18 pm    Post subject: How does #define operate ? Hello If I say #define MASK 0x00F0 #define BIT_SET MASK #define MASK 0x000F than what value will BIT_SET macro expand to ? I mean the preprocessor does lasy evaluation or immediate evaluation ? I tryed to figure it out from MSDN once but I couldn't Thank you Timothy Madden Romania ---------------------------------------------------------------------------- And I don't wanna miss a thing Back to top Howard Guest Posted: Mon Sep 27, 2004 2:40 pm    Post subject: Re: How does #define operate ? "Timothy Madden" <batman (AT) rmv (DOT) spam.home.ro> wrote Quote: Hello If I say #define MASK 0x00F0 #define BIT_SET MASK #define MASK 0x000F than what value will BIT_SET macro expand to ? I mean the preprocessor does lasy evaluation or immediate evaluation ? I tryed to figure it out from MSDN once but I couldn't I don't think the standard deals with preprocessor stuff, but on my compiler it gives me a redefinition error. If I want to re-#define something, I have to #undef it first. Which would imply (assuming you threw in a #undef MASK before the second #define) that BIT_SET would still be 0x00F0, because otherwise what would the point be of requiring #undef? I see several places where something is defined just before including a file that needs it, then undefined so as not to interfere with other, later, includes. -Howard Back to top Xenos Guest Posted: Mon Sep 27, 2004 4:44 pm    Post subject: Re: How does #define operate ? "Howard" <alicebt (AT) hotmail (DOT) com> wrote Quote: "Timothy Madden" <batman (AT) rmv (DOT) spam.home.ro> wrote in message news:2rqlopF1dg1v2U1 (AT) uni-berlin (DOT) de... Hello If I say #define MASK 0x00F0 #define BIT_SET MASK #define MASK 0x000F than what value will BIT_SET macro expand to ? I mean the preprocessor does lasy evaluation or immediate evaluation ? I tryed to figure it out from MSDN once but I couldn't I don't think the standard deals with preprocessor stuff, but on my compiler it gives me a redefinition error. If I want to re-#define something, I have to #undef it first. Which would imply (assuming you threw in a #undef MASK before the second #define) that BIT_SET would still be 0x00F0, because otherwise what would the point be of requiring #undef? I see several places where something is defined just before including a file that needs it, then undefined so as not to interfere with other, later, includes. -Howard Not true, BIT_SET expands to MASK, not 0x00F0. If you redefine MASK, your redefine BIT_SET. DrX Back to top Howard Guest Posted: Mon Sep 27, 2004 8:20 pm    Post subject: Re: How does #define operate ? "Xenos" <dont.spam.me (AT) spamhate (DOT) com> wrote Quote: "Howard" <alicebt (AT) hotmail (DOT) com> wrote in message news:bDV5d.441932$OB3.21760 (AT) bgtnsc05-news (DOT) ops.worldnet.att.net... "Timothy Madden" <batman (AT) rmv (DOT) spam.home.ro> wrote in message news:2rqlopF1dg1v2U1 (AT) uni-berlin (DOT) de... Hello If I say #define MASK 0x00F0 #define BIT_SET MASK #define MASK 0x000F than what value will BIT_SET macro expand to ? I mean the preprocessor does lasy evaluation or immediate evaluation ? I tryed to figure it out from MSDN once but I couldn't I don't think the standard deals with preprocessor stuff, but on my compiler it gives me a redefinition error. If I want to re-#define something, I have to #undef it first. Which would imply (assuming you threw in a #undef MASK before the second #define) that BIT_SET would still be 0x00F0, because otherwise what would the point be of requiring #undef? I see several places where something is defined just before including a file that needs it, then undefined so as not to interfere with other, later, includes. -Howard Not true, BIT_SET expands to MASK, not 0x00F0. If you redefine MASK, your redefine BIT_SET. DrX You're correct (at least using CodeWarrior on the Mac). And it's obvious why. My thinking was backwards. At the time/place of expansion, it evaluates the symbol, and by that time the symbol has changed. (But I definitely do need to #undef it before I can #define it again.) -Howard Back to top Timothy Madden Guest Posted: Tue Sep 28, 2004 6:39 am    Post subject: Re: How does #define operate ? "Howard" <alicebt (AT) hotmail (DOT) com> wrote Quote: "Xenos" <dont.spam.me (AT) spamhate (DOT) com> wrote in message news:cj9g21$gi52 (AT) cui1 (DOT) lmms.lmco.com... "Howard" <alicebt (AT) hotmail (DOT) com> wrote in message news:bDV5d.441932$OB3.21760 (AT) bgtnsc05-news (DOT) ops.worldnet.att.net... "Timothy Madden" <batman (AT) rmv (DOT) spam.home.ro> wrote in message news:2rqlopF1dg1v2U1 (AT) uni-berlin (DOT) de... Hello If I say #define MASK 0x00F0 #define BIT_SET MASK #define MASK 0x000F than what value will BIT_SET macro expand to ? I mean the preprocessor does lasy evaluation or immediate evaluation ? I tryed to figure it out from MSDN once but I couldn't I don't think the standard deals with preprocessor stuff, but on my compiler it gives me a redefinition error. If I want to re-#define something, I have to #undef it first. Which would imply (assuming you threw in a #undef MASK before the second #define) that BIT_SET would still be 0x00F0, because otherwise what would the point be of requiring #undef? I see several places where something is defined just before including a file that needs it, then undefined so as not to interfere with other, later, includes. -Howard Not true, BIT_SET expands to MASK, not 0x00F0. If you redefine MASK, your redefine BIT_SET. DrX You're correct (at least using CodeWarrior on the Mac). And it's obvious why. My thinking was backwards. At the time/place of expansion, it evaluates the symbol, and by that time the symbol has changed. (But I definitely do need to #undef it before I can #define it again.) Actualy I really meant: #define MASK 0x00F0 #define BIT_SET MASK #undef MASK #define MASK 0x000F Should the standard not say something about this ? Could MASK in turn be defined as some other defined symbol ? How many levels can this go ? Why does any book not say anything about it ? Anyone can tell me how other compilers behave ? Thank you Timothy Madden Romania ------------------------------------------------- And I don't wanna miss a thing Back to top Xenos Guest Posted: Tue Sep 28, 2004 11:41 am    Post subject: Re: How does #define operate ? "Timothy Madden" <batman (AT) rmv (DOT) spam.home.ro> wrote Quote: Actualy I really meant: #define MASK 0x00F0 #define BIT_SET MASK #undef MASK #define MASK 0x000F Should the standard not say something about this ? Could MASK in turn be defined as some other defined symbol ? How many levels can this go ? Why does any book not say anything about it ? Anyone can tell me how other compilers behave ? It's not the compiler, but the preprocessor. They behave by following the standard. You are not setting one variable equal to another. You are telling the preprocessor that the macro BIT_SET is equal to the the text "MASK" (without the quotes). After the text replacement, the code will be rescanned looking for more macros. If MASK is defined when this is done, then MASK will be replaced. If not, it will be passed as is to the compiler. DrX 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 Page 1 of 1   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.