C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  #pragma STDC ONCE: interest? Goto page 1, 2, 3, 4, 5, 6, 7  Next          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message Gennaro Prota Guest Posted: Wed Dec 06, 2006 12:39 am    Post subject: #pragma STDC ONCE: interest? Hi, given that there will be much rewording and expansion in clause 16 of the standard to bring it in synch with C99 I was thinking to submit a proposal for a "pragma once" directive, in the form #pragma STDC ONCE I'd like to know, however, if committee members here feel that the proposal will have a concrete chance to be considered. -- Gennaro Prota. C++ developer. For hire. (to mail me, remove any 'u' from the address) --- [ 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 Pete Becker Guest Posted: Wed Dec 06, 2006 12:39 am    Post subject: Re: #pragma STDC ONCE: interest? Gennaro Prota wrote: Quote: given that there will be much rewording and expansion in clause 16 of the standard to bring it in synch with C99 Those changes have already been made. See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1653.htm. Quote: I was thinking to submit a proposal for a "pragma once" directive, in the form #pragma STDC ONCE I'd like to know, however, if committee members here feel that the proposal will have a concrete chance to be considered. I'm under the impression that everyone who has implemented this has come to the conclusion that it's not worth the bother. But aside from technical merits, the deadline for new submissions for C++0x has passed, so I doubt that a new proposal would get much consideration. For an overview of the status of the various extension proposals for C++0x see http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2122.htm. -- -- Pete Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The Standard C++ Library Extensions: a Tutorial and Reference." (www.petebecker.com/tr1book) --- [ 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 Gennaro Prota Guest Posted: Wed Dec 06, 2006 12:39 am    Post subject: Re: #pragma STDC ONCE: interest? On Tue, 5 Dec 2006 15:05:30 CST, Pete Becker wrote: Quote: I'm under the impression that everyone who has implemented this has come to the conclusion that it's not worth the bother. But aside from technical merits, the deadline for new submissions for C++0x has passed, so I doubt that a new proposal would get much consideration. The advantage is for the user, as it relieves from inventing a name for the include guard macro. Also it is so simple to specify and implement... Perhaps for C++1x, then. -- Gennaro Prota. C++ developer. For hire. (to mail me, remove any 'u' from the address) --- [ 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 peter koch Guest Posted: Wed Dec 06, 2006 4:52 am    Post subject: Re: #pragma STDC ONCE: interest? Gennaro Prota skrev: Quote: Hi, given that there will be much rewording and expansion in clause 16 of the standard to bring it in synch with C99 I was thinking to submit a proposal for a "pragma once" directive, in the form #pragma STDC ONCE I'd like to know, however, if committee members here feel that the proposal will have a concrete chance to be considered. -- Gennaro Prota. C++ developer. For hire. (to mail me, remove any 'u' from the address) I don't really see the purpose of that pragma in the first place, but as you are doubtlessly more knowledgeable than me in this area, lets ignore that and assume that pragma once is useful. In that case, why not simply use the de facto standard format - #pragma once. Looks nicer that way, I think. /Peter --- [ 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 Pete Becker Guest Posted: Wed Dec 06, 2006 4:59 am    Post subject: Re: #pragma STDC ONCE: interest? Gennaro Prota wrote: Quote: The advantage is for the user, as it relieves from inventing a name for the include guard macro. Also it is so simple to specify and implement... Perhaps for C++1x, then. It is not simple to specify, nor is it simple to implement (if it were, the simplistic analysis on Wikipedia wouldn't have a list of compilers that got it wrong). Remember, an included file can be out on a network somewhere, on an unknown file system, accessible through multiple paths, with different names. -- -- Pete Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The Standard C++ Library Extensions: a Tutorial and Reference." (www.petebecker.com/tr1book) --- [ 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 Gennaro Prota Guest Posted: Wed Dec 06, 2006 6:22 am    Post subject: Re: #pragma STDC ONCE: interest? On Tue, 5 Dec 2006 22:59:35 GMT, Pete Becker wrote: Quote: Gennaro Prota wrote: The advantage is for the user, as it relieves from inventing a name for the include guard macro. Also it is so simple to specify and implement... Perhaps for C++1x, then. It is not simple to specify, nor is it simple to implement (if it were, the simplistic analysis on Wikipedia wouldn't have a list of compilers that got it wrong). Hmm, could you please provide a link? I can't find any article with such a list. Quote: Remember, an included file can be out on a network somewhere, on an unknown file system, accessible through multiple paths, with different names. I know. But the EDG front-end, and gcc, claim to recognize the include guard idiom. How can they implement it correctly? -- Gennaro Prota. C++ developer. For hire. (to mail me, remove any 'u' from the address) --- [ 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 Seungbeom Kim Guest Posted: Wed Dec 06, 2006 6:46 am    Post subject: Re: #pragma STDC ONCE: interest? Gennaro Prota wrote: Quote: On Tue, 5 Dec 2006 15:05:30 CST, Pete Becker wrote: I'm under the impression that everyone who has implemented this has come to the conclusion that it's not worth the bother. But aside from technical merits, the deadline for new submissions for C++0x has passed, so I doubt that a new proposal would get much consideration. The advantage is for the user, as it relieves from inventing a name for the include guard macro. Also it is so simple to specify and implement... Perhaps for C++1x, then. Even if it were admitted into C++0x or C++1x, you would still have to write the include guards yourself anyway in any serious program to support old compilers, wouldn't you? And you couldn't argue otherwise because supporting old compilers is trivially easy (3 more lines). Then I don't see much benefit in such a change. Maybe it's way easier just to write a program that generates the include guards. $ genhdr headername.h $ cat headername.h #ifndef HEADERNAME_H_INCLUDED #define HEADERNAME_H_INCLUDED #endif $ _ Or there probably are ones already. It's just that writing one myself or even searching for a good one costs more than writing out the include guard when I need it. :) -- Seungbeom Kim --- [ 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 Nevin :-] Liber Guest Posted: Wed Dec 06, 2006 6:46 am    Post subject: Re: #pragma STDC ONCE: interest? In article <5d2cn2tvg6te1r2h5otocpmsec5e9gl2sk (AT) 4ax (DOT) com>, geunnaro_prouta (AT) yahoo (DOT) com (Gennaro Prota) wrote: Quote: I know. But the EDG front-end, and gcc, claim to recognize the include guard idiom. How can they implement it correctly? The include guard idiom is based on macro symbols, not on whether or not two files are the same file. Information about the latter must be provided by the operating system. -- Nevin "" Liber <mailto:nevin (AT) eviloverlord (DOT) com> --- [ 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 James Kanze Guest Posted: Wed Dec 06, 2006 4:07 pm    Post subject: Re: #pragma STDC ONCE: interest? Seungbeom Kim wrote: Quote: Gennaro Prota wrote: On Tue, 5 Dec 2006 15:05:30 CST, Pete Becker wrote: I'm under the impression that everyone who has implemented this has come to the conclusion that it's not worth the bother. But aside from technical merits, the deadline for new submissions for C++0x has passed, so I doubt that a new proposal would get much consideration. The advantage is for the user, as it relieves from inventing a name for the include guard macro. Also it is so simple to specify and implement... Perhaps for C++1x, then. Even if it were admitted into C++0x or C++1x, you would still have to write the include guards yourself anyway in any serious program to support old compilers, wouldn't you? And you couldn't argue otherwise because supporting old compilers is trivially easy (3 more lines). Then I don't see much benefit in such a change. Maybe it's way easier just to write a program that generates the include guards. $ genhdr headername.h $ cat headername.h #ifndef HEADERNAME_H_INCLUDED #define HEADERNAME_H_INCLUDED #endif $ _ Or there probably are ones already. It's just that writing one myself or even searching for a good one costs more than writing out the include guard when I need it. Does anyone use an editor which doesn't do this (and insert other boilerplate text as well) whenever you open a file which doesn't exist and whose name ends in .h, .hh, .hpp, etc.? It seems like a standard feature, present in all of the editors I use (vim, emacs) except the original vi. Of course, you have to configure it to do so, but I would expect this to be the case in any normal development environment---you also want boilerplate copyright notices, and who knows what all else. (I've found it useful to put // Local Variables: --- for emacs // mode: c++ --- for emacs // tab-width: 8 --- for emacs // End: --- for emacs // vim: set ts=8 sw=4 filetype=cpp: --- for vim at the end of all of my sources, for example. And you should see what gets generated if the filename ends with .html:-).) If you don't know how to do this with your current editor, ask in the appropriate forum for that editor. You'll typically find a package already available which does most of the work. (I forget what I used with emacs, but I know that I didn't have to write a single line of elisp:-).) Of course, in a lot of cases, your source code management system will take care of these details, so you don't have to bother configuring the editor. -- James Kanze (GABI Software) email:james.kanze (AT) gmail (DOT) com 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 James Kanze Guest Posted: Wed Dec 06, 2006 5:45 pm    Post subject: Re: #pragma STDC ONCE: interest? Nevin :-] Liber wrote: Quote: In article <5d2cn2tvg6te1r2h5otocpmsec5e9gl2sk (AT) 4ax (DOT) com>, geunnaro_prouta (AT) yahoo (DOT) com (Gennaro Prota) wrote: I know. But the EDG front-end, and gcc, claim to recognize the include guard idiom. How can they implement it correctly? The include guard idiom is based on macro symbols, not on whether or not two files are the same file. Information about the latter must be provided by the operating system. The optimization based on recognizing it is based on recognizing that two includes actually include the same file. And the reason that it can be implemented correctly is that it isn't an error if they do happen to read the same file twice; it just results in a slower compile time. So they can take a conservative strategy, and in case of doubt, skip the optimization. This would not be the case in the case of #pragma once. -- James Kanze (GABI Software) email:james.kanze (AT) gmail (DOT) com 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 Peter Steiner Guest Posted: Wed Dec 06, 2006 5:46 pm    Post subject: Re: #pragma STDC ONCE: interest? Seungbeom Kim wrote: Quote: Then I don't see much benefit in such a change. Maybe it's way easier just to write a program that generates the include guards. besides the improvement in usability, a pragma once statements decreases preprocessor runtime cost. include guards don't allow the preprocessor to omit the file for obvious reasons, while pragma once does so. this can result in a significant preprocessor speed up. -- peter --- [ 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 peter koch Guest Posted: Wed Dec 06, 2006 11:23 pm    Post subject: Re: #pragma STDC ONCE: interest? Gennaro Prota skrev: Quote: On Tue, 5 Dec 2006 22:52:31 CST, peter koch wrote: [ #pragma STDC ONCE ] I'd like to know, however, if committee members here feel that the [snip] why not simply use the de facto standard format - #pragma once. Looks nicer that way, I think. I now understand that the "STDC" form of the pragma is for "standardised" pragmas and would thus support that form as well. Quote: My favourite form would actually be: #once There's no need, I think, to bring "pragma" into it. As I said, the usefulness of the construct lies in the fact that it doesn't require any user-defined identifier. (In retrospect, I guess the include once behavior should have been the default, with a directive to request otherwise, but that's (perhaps) too late to change, especially now that many "recursive inclusion" techniques have been popularized by the boost preprocessor library) The names of my controlling macros usually encode the project name, the filename, the developer's name and the date of creation of the file, and are automatically generated, but still that's not enough to avoid any reasonable conflict. My controlling macro once had a tail of "blind typing" so it looked somewhat like #if !defined INCLUDE_HPPlsfvugasovytgsurkxjtlouejrasvnyhjsytdjcyvlistcge #define INCLUDE_HPPlsfvugasovytgsurkxjtlouejrasvnyhjsytdjcyvlistcge much to the amusement of my fellow collegues. I fell victim for their teasing and now follow your way (including also the time of day). I doubt the likelyhood of having identical guards is more than hypothetical. One more serious consideration is what to do with linked files. Is it always possible to determine that a headerfile has already been drawn in when the name is different? I would not bet on that and would thus prefer the good old guards. Kind regards Peter --- [ 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 Gennaro Prota Guest Posted: Wed Dec 06, 2006 11:42 pm    Post subject: Re: #pragma STDC ONCE: interest? On Tue, 5 Dec 2006 22:52:31 CST, peter koch wrote: Quote: [ #pragma STDC ONCE ] I'd like to know, however, if committee members here feel that the proposal will have a concrete chance to be considered. I don't really see the purpose of that pragma in the first place, but as you are doubtlessly more knowledgeable than me in this area, Not at all :-) Quote: lets ignore that and assume that pragma once is useful. In that case, why not simply use the de facto standard format - #pragma once. Looks nicer that way, I think. My favourite form would actually be: #once There's no need, I think, to bring "pragma" into it. As I said, the usefulness of the construct lies in the fact that it doesn't require any user-defined identifier. (In retrospect, I guess the include once behavior should have been the default, with a directive to request otherwise, but that's (perhaps) too late to change, especially now that many "recursive inclusion" techniques have been popularized by the boost preprocessor library) The names of my controlling macros usually encode the project name, the filename, the developer's name and the date of creation of the file, and are automatically generated, but still that's not enough to avoid any reasonable conflict. -- Gennaro Prota. C++ developer. For hire. (to mail me, remove any 'u' from the address) --- [ 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 Ben Hutchings Guest Posted: Thu Dec 07, 2006 5:23 am    Post subject: Re: #pragma STDC ONCE: interest? On 2006-12-06, Peter Steiner <pnsteiner (AT) gmail (DOT) com> wrote: Quote: Seungbeom Kim wrote: Then I don't see much benefit in such a change. Maybe it's way easier just to write a program that generates the include guards. besides the improvement in usability, a pragma once statements decreases preprocessor runtime cost. include guards don't allow the preprocessor to omit the file for obvious reasons, while pragma once does so. snip #pragma once has been suggested many times, but it's difficult to specify and may be impossible to implement on some common systems. I know a popular compiler that implements #pragma once which allows a file containing #pragma once that's on a case-insensitive file-system to be included more than once if the include directives use differing capitalisation, Another popular compiler recognises the #ifndef FOO...#define FOO... #endif pattern and avoids repeatedly reading such files (so long as it recognises them) if the controlling macro is still defined. Even if it fails to recognise two paths as pointing to the same file, this only makes compilation a little slower. I know which behaviour I prefer. Ben. -- Ben Hutchings Never attribute to conspiracy what can adequately be explained by stupidity. --- [ 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 eric_backus@alum.mit.edu Guest Posted: Thu Dec 07, 2006 7:21 am    Post subject: Re: #pragma STDC ONCE: interest? peter koch wrote: Quote: One more serious consideration is what to do with linked files. Is it always possible to determine that a headerfile has already been drawn in when the name is different? I would not bet on that and would thus prefer the good old guards. Which failure is more likely: * Your system supports linked files, the include files in your project make use of that, but the compiler can't figure out that two files are linked together, or * You use good old include guards, but accidentally use the same include guard in different include files, due to using cut-and-paste to create one of the files. I'm pretty sure the second is much more likely--I've seen it happen. To me, the issue is not that include guards are trivially easy for people to use (they are), the issue is that people occasionally make mistakes and duplicate the include guards, and that include guards by definition pollute the global namespace. I understand the difficulties in determining file identity when you involve networking, hard and soft links, and so on. But I would think the standard could just say that file identity is determined in an implementation defined way, and leave this as a quality of implementation issue. --- [ comp.std.c++ is moderated. 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