C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Preprocessor Push/Pop Idiom, How To? Goto page 1, 2  Next          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message new.netins.net Guest Posted: Fri Sep 29, 2006 10:09 pm    Post subject: Preprocessor Push/Pop Idiom, How To? I wan't to save the current value of a manifest constant on a stack, and set the value to another value. Later, I want to restore the previous value. I'd like this ability to nest. I.e. I want a preprocessor stack. Imagine preprocessor directives #push and #pop, such that: #define MY_VALUE 10 // MY_VALUE == 10 #push MY_VALUE 20 // now MY_VALUE == 20 #push MY_VALUE 30 // MY_VALUE == 30, ... #pop MY_VALUE // restored, MY_VALUE #pop MY_VALUE // MY_VALUE = 10 #pop MY_VALUE // undefined, could be not define, could be 10, dunno How can I approximate this behavior? terry -- [ 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: Sat Sep 30, 2006 12:02 am    Post subject: Re: Preprocessor Push/Pop Idiom, How To? new.netins.net wrote: Quote: How can I approximate this behavior? Use const variables and scopes: const int MY_VALUE = 10; { const int MY_VALUE = 20; { const int MY_VALUE = 30; } // MY_VALUE = 20 } // MY_VALUE = 10 -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Terry G Guest Posted: Sat Sep 30, 2006 3:34 am    Post subject: Re: Preprocessor Push/Pop Idiom, How To? "Ron Natalie" <ron (AT) spamcop (DOT) net> wrote in message news:451d5f93$0$24194$9a6e19ea (AT) news (DOT) newshosting.com... Quote: new.netins.net wrote: Use const variables and scopes: Thank you for your help. I need everything to be independent of scope. I really want MY_VALUE to be a manifest constant and not a "const int". The reason for this is that its used in another macro. The idea is that MY_VALUE is a default that is applied to an entire compilation unit (.h or .cpp file). I probably over-simplified my code for the example. Here's more over-simplified code. #define SHORTHAND MyTemplate<MY_VALUE, long, 32> If the user writes #define MY_VALUE 10 SHORTHAND a; #push MY_VALUE 20 SHORTHAND b; #push MY_VALUE 30 SHORTHAND c; #pop MY_VALUE SHORTHAND d; #pop MY_VALUE SHORTHAND e; The code emitted after the preprocessing phase should be: MyTemplate<10, long, 32> a; MyTemplate<20, long, 32> b; MyTemplate<30, long, 32 > c; MyTemplate<20, long, 32> d; MyTemplate<10, long, 32> e; If this helps, replace SHORTHAND with INT32, MyTemplate with Endian, MY_VALUE with ENDIAN_DEFAULT, and (10, 20, 30) with (LITTLE_ENDIAN, BIG_ENDIAN, and NATIVE_ENDIAN). terry -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Frederick Gotham Guest Posted: Sat Sep 30, 2006 6:47 am    Post subject: Re: Preprocessor Push/Pop Idiom, How To? Ron Natalie posted: Quote: const int MY_VALUE = 10; I suggest that ALL CAPS be used for macros and macros only. -- Frederick Gotham -- [ 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: Sat Sep 30, 2006 7:57 pm    Post subject: Re: Preprocessor Push/Pop Idiom, How To? Frederick Gotham wrote: Quote: Ron Natalie posted: const int MY_VALUE = 10; I suggest that ALL CAPS be used for macros and macros only. I suggest that you you are outdated. I suggest that using all caps for manifest concepts is as valid. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top James Dennett Guest Posted: Sun Oct 01, 2006 1:44 am    Post subject: Re: Preprocessor Push/Pop Idiom, How To? Ron Natalie wrote: Quote: Frederick Gotham wrote: Ron Natalie posted: const int MY_VALUE = 10; I suggest that ALL CAPS be used for macros and macros only. I suggest that you you are outdated. I suggest that using all caps for manifest concepts is as valid. I suggest that the use of all caps for non-macro constants is outdated, and that the reasons for segregating the identifier namespace into APP_CAPS for macros (only) and all other identifiers for non-macros is based on good reasoning that has not diminished at all. (It's unfortunate that other languages have picked up on this and used ALL_CAPS for certain kinds of constants even though they don't have a C-like preprocessor to work around. APP_CAPS is not particularly readable. But that's an aside.) I see current best practice as reserving use of ALL_CAPS for macros, and that has become more, not less, true over the last decade. There are still an unfortunate number of texts (usually not the good ones, but sometimes even there) which don't follow this sensible convention. -- James -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Frederick Gotham Guest Posted: Sun Oct 01, 2006 1:45 am    Post subject: Re: Preprocessor Push/Pop Idiom, How To? Ron Natalie posted: Quote: I suggest that ALL CAPS be used for macros and macros only. I suggest that you are outdated. I (or my methods, more like) will be outdated when macros have been done away with, which, by the looks of things, won't happen any time soon. Quote: I suggest that using all caps for manifest concepts is as valid. Macros work slightly different from your usual functions, objects and typedefs. For instance, take this macro function: #define SQR(x) ((x)*(x)) This won't work in the way we would normally expect a function to work: SQR(i++); /* Double-evaluation of the argument */ int (*p)(int) = SQR; /* Can't get a function pointer */ Also, consider the following macro typedef: #define MY_TYPE int* This macro won't work properly when used in such situations as: void Func(const MY_TYPE arg) {} This is a problem. A big problem. We need a reminder, something to pinch us each and every time we make use of a macro -- Usher in ALL CAPS for macros and for macros only. If we were to start using ALL CAPS for things other than macros, then we'd have a "Boy Who Cried Wolf" situation -- the pinch would get duller and duller each time we deal with a non-macro... until eventually we don't pay a blind bit of notice to ALL CAPS and we drop our level of caution, leading to such mistakes as: SQR(i++); The use of ALL CAPS for anything other than macros should be shunned. -- Frederick Gotham [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Alf P. Steinbach Guest Posted: Sun Oct 01, 2006 3:03 am    Post subject: Re: Preprocessor Push/Pop Idiom, How To? * Ron Natalie: Quote: Frederick Gotham wrote: Ron Natalie posted: const int MY_VALUE = 10; I suggest that ALL CAPS be used for macros and macros only. I suggest that you you are outdated. I suggest that using all caps for manifest concepts is as valid. You probably mean constants, not concepts. The main rationale for the all caps convention for macros is that macros don't respect scopes: we want to minimize name collisions. Therefore, in original C, which lacked 'const', constants defined as macros usually had all caps names. That in-effect convention was picked up by programmers using other languages, most notably Java, which lacks a preprocessor, and there used in a way incompatible with the original goal of supporting use of the preprocessor. C++ is the direct descendant of the language where that convention originated. In C++ the convention /in its original useful form/ is still very much alive. Bringing the bastardized incompatible Java form of that convention back into C++ would be, well, incongruous... -- A: Because it messes up the order in which people normally read text. Q: Why is it such a bad thing? A: Top-posting. Q: What is the most annoying thing on usenet and in e-mail? -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Terry G Guest Posted: Sun Oct 01, 2006 2:59 pm    Post subject: Re: Preprocessor Push/Pop Idiom, How To? "new.netins.net" <tjgolubi (AT) netins (DOT) net> wrote in message news:efjh9f$2a$1 (AT) news (DOT) netins.net... Quote: I want to save the current value of a manifest constant on a stack, and set the value to another value. Later, I want to restore the previous value. I'd like this ability to nest. I.e. I want a preprocessor stack. Imagine preprocessor directives #push and #pop, such that: #define MY_VALUE 10 // MY_VALUE == 10 #push MY_VALUE 20 // now MY_VALUE == 20 #push MY_VALUE 30 // MY_VALUE == 30, ... #pop MY_VALUE // restored, MY_VALUE #pop MY_VALUE // MY_VALUE = 10 #pop MY_VALUE // undefined, could be not define, could be 10, dunno How can I approximate this behavior? Here is a solution that DOES NOT WORK, but it represents what I wish could be done. #define MY_VALUE 10 #define MY_VALUE_temp MY_VALUE #undef MY_VALUE #define MY_VALUE 20 // Use MY_VALUE with value of 20. #undef MY_VALUE #define MY_VALUE MY_VALUE_temp #undef MY_VALUE_temp // MY_VALUE is restored to 10. Again, I know this doesn't work, but I'm looking for a way to accomplish the same thing. It seems impossible. Would an extension to the C preprocessor to accomplish this be a good idea? Surely someone has proposed this before. #define MY_VALUE 10 #eval MY_VALUE_temp MY_VALUE // Expands right hand side before assigning to MY_VALUE_temp. #undef MYVALUE #define MY_VALUE 20 // Use MY_VALUE with value of 20. #undef MY_VALUE #eval MY_VALUE MY_VALUE_temp #undef MY_VALUE_temp // MY_VALUE is restored to 10. #eval would be like := in gnu makefiles. #swap X Y might be useful too. I don't think I'd like #push/#pop though. Any thoughts? terry -- [ 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: Sun Oct 01, 2006 7:57 pm    Post subject: Re: Preprocessor Push/Pop Idiom, How To? In article <4o7h4lFdc1ofU1 (AT) individual (DOT) net>, Alf P. Steinbach <alfps (AT) start (DOT) no> writes Quote: C++ is the direct descendant of the language where that convention originated. In C++ the convention /in its original useful form/ is still very much alive. Bringing the bastardized incompatible Java form of that convention back into C++ would be, well, incongruous... Yes, I completely agree, but I suspect there is another reason that ALL_CAPS has crept into use for non-preprocessor names. I think that many early adopters of C++ did not really understand why ALL_CAPS was used in C for manifest constants. When they converted from using the pre-processor to using const type definitions for manifest constants they just transferred the original name. Of course there was another pressure to do this, it meant that legacy C code could be 'improved' by replacing #define VAR 3 with int const VAR(3); and all the uses would remain working. I understand the reasoning here but think that it is misplaced, just leave the original and the code will work as intended. What really irritates the h*** out of me is code such as: enum colour {RED, BLUE, GREEN}; enumerators were never written as all caps in C (or not by programmers who understood the divide between the pre-processor and other names) but, of course, once you start using all caps for manifest constants you obviously should do the same for enumeration constants. Learn why things are done rather than simply see the superficial and you will write code that is less vulnerable to the pre-processor. -- Francis Glassborow ACCU Author of 'You Can Do It!' and "You Can Program in C++" 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: Sun Oct 01, 2006 7:57 pm    Post subject: Re: Preprocessor Push/Pop Idiom, How To? In article <efn30e$5as$1 (AT) news (DOT) netins.net>, Terry G <tjgolubi (AT) netins (DOT) net> writes Quote: Here is a solution that DOES NOT WORK, but it represents what I wish could be done. #define MY_VALUE 10 #define MY_VALUE_temp MY_VALUE #undef MY_VALUE #define MY_VALUE 20 // Use MY_VALUE with value of 20. #undef MY_VALUE #define MY_VALUE MY_VALUE_temp #undef MY_VALUE_temp // MY_VALUE is restored to 10. Again, I know this doesn't work, but I'm looking for a way to accomplish the same thing. It seems impossible. My first reaction is 'excellent' the last thing I want in code is constants that vary :) The next reaction is that C++ programmers should be avoiding the use of the pre-processor where possible because its use is full of unexpected problems once you are writing more than trivial student code. The biggest problem for C++ where real programs are often many tens or even hundreds of thousands of lines long is the complete lack of any scope for the pre-processor. Now if you explain what the problem is that you think the above might solve, then we might be able to suggest a solution. For now I find it hard to understand why you want a variable constant :) -- Francis Glassborow ACCU Author of 'You Can Do It!' and "You Can Program in C++" 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 Gennaro Prota Guest Posted: Sun Oct 01, 2006 7:58 pm    Post subject: Re: Preprocessor Push/Pop Idiom, How To? On 1 Oct 2006 05:59:17 -0400, "Terry G" <tjgolubi (AT) netins (DOT) net> wrote: Quote: [...] Here is a solution that DOES NOT WORK, but it represents what I wish could be done. #define MY_VALUE 10 #define MY_VALUE_temp MY_VALUE #undef MY_VALUE #define MY_VALUE 20 // Use MY_VALUE with value of 20. #undef MY_VALUE #define MY_VALUE MY_VALUE_temp #undef MY_VALUE_temp // MY_VALUE is restored to 10. Again, I know this doesn't work, but I'm looking for a way to accomplish the same thing. It seems impossible. I'm not sure I understand the details of what "the same thing" is (which is crucial in this case) but... did you consider Boost.Preprocessor's evaluated slots? -- Gennaro Prota -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Jeffrey Schwab Guest Posted: Mon Oct 02, 2006 5:53 am    Post subject: Re: Preprocessor Push/Pop Idiom, How To? Alf P. Steinbach wrote: Quote: * Ron Natalie: Frederick Gotham wrote: Ron Natalie posted: const int MY_VALUE = 10; I suggest that ALL CAPS be used for macros and macros only. I suggest that you you are outdated. I suggest that using all caps for manifest concepts is as valid. You probably mean constants, not concepts. The main rationale for the all caps convention for macros is that macros don't respect scopes: we want to minimize name collisions. Therefore, in original C, which lacked 'const', constants defined as macros usually had all caps names. That in-effect convention was picked up by programmers using other languages, most notably Java, which lacks a preprocessor, and there used in a way incompatible with the original goal of supporting use of the preprocessor. C++ is the direct descendant of the language where that convention originated. In C++ the convention /in its original useful form/ is still very much alive. Bringing the bastardized incompatible Java form of that convention back into C++ would be, well, incongruous... "Bastardized" is a little harsh. While I agree that ALL_CAPS should only be used for pre-processor macros in C and C++, it is very useful in Java for making constants visually obvious. When I see an identifier in Java, I ordinarily don't know who has modified this particular member variable, and I have to consider that it might have different values (and mean different things) at various points in the code. On the other hand, an IDENTIFIER is clearly a public static final constant, and a reader can see immediately that the actual value doesn't matter as much as the fact that it is unique. Different language often use the same convention to mean different things, and that's OK. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Bart.van.Ingen.Schenau@ic Guest Posted: Mon Oct 02, 2006 9:53 pm    Post subject: Re: Preprocessor Push/Pop Idiom, How To? Terry G wrote: Quote: "Ron Natalie" <ron (AT) spamcop (DOT) net> wrote in message news:451d5f93$0$24194$9a6e19ea (AT) news (DOT) newshosting.com... new.netins.net wrote: Use const variables and scopes: Thank you for your help. I need everything to be independent of scope. I really want MY_VALUE to be a manifest constant and not a "const int". The reason for this is that its used in another macro. The idea is that MY_VALUE is a default that is applied to an entire compilation unit (.h or .cpp file). I probably over-simplified my code for the example. Here's more over-simplified code. #define SHORTHAND MyTemplate<MY_VALUE, long, 32 If the user writes #define MY_VALUE 10 SHORTHAND a; #push MY_VALUE 20 SHORTHAND b; #push MY_VALUE 30 SHORTHAND c; #pop MY_VALUE SHORTHAND d; #pop MY_VALUE SHORTHAND e; The code emitted after the preprocessing phase should be: MyTemplate<10, long, 32> a; MyTemplate<20, long, 32> b; MyTemplate<30, long, 32 > c; MyTemplate<20, long, 32> d; MyTemplate<10, long, 32> e; How about adding a parameter to your SHORTHAND macro: #define SHORTHAND(value) MyTemplate<value, long, 32> This can be used as: SHORTHAND(10) a; SHORTHAND(20) b; SHORTHAND(30) c; SHORTHAND(20) d; SHORTHAND(10) e; and will come out exactly as the expected result you give above. Quote: If this helps, replace SHORTHAND with INT32, MyTemplate with Endian, MY_VALUE with ENDIAN_DEFAULT, and (10, 20, 30) with (LITTLE_ENDIAN, BIG_ENDIAN, and NATIVE_ENDIAN). terry Bart v Ingen Schenau -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Jason Hise Guest Posted: Mon Oct 02, 2006 10:11 pm    Post subject: Re: Preprocessor Push/Pop Idiom, How To? If you are ok with something compiler specific: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vclang/html /vcrefpop_macro.asp -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] 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 (Moderated) All times are GMT Goto page 1, 2  Next Page 1 of 2   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.