C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  embedded '' in comment          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message bloodymir Guest Posted: Wed Oct 05, 2005 9:33 pm    Post subject: embedded '\' in comment Hi! Recently I had a problem with an array initialization. After quite some testing I found the compiler (gcc3) was missing a line. I tracked down the problem to a "//" comment on the line before which ends on double backslash. It works like this: int main() { int i = 0; // \ ++i; cout << i; } So shall "++i" be part of the comment or not? Frank [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Markus Moll Guest Posted: Thu Oct 06, 2005 1:29 pm    Post subject: Re: embedded '\' in comment Hi bloodymir wrote: Quote: int main() { int i = 0; // \ ++i; cout << i; } So shall "++i" be part of the comment or not? Yes, your compiler is right. Markus [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Alberto Ganesh Barbati Guest Posted: Thu Oct 06, 2005 1:34 pm    Post subject: Re: embedded '\' in comment bloodymir wrote: Quote: Hi! Recently I had a problem with an array initialization. After quite some testing I found the compiler (gcc3) was missing a line. I tracked down the problem to a "//" comment on the line before which ends on double backslash. It works like this: int main() { int i = 0; // \ ++i; cout << i; } So shall "++i" be part of the comment or not? Yes, "++i" is part of the comment. If a line (comment or not, doesn't matter) ends with a backslash then the newline and the backslash are ignored in translation phase 2 and the two "physical lines" are merged to form a single "logical line" (§2.1/2). The presence of two backslashes in your example is an accident, only one would suffice. In this particular case the two physical lines: ------ // \ ++i; ------ are interpreted as ------ // ++i; ------ (notice that the leading blanks of the second line are still kept in this translation phase). This rules applies to comments also because comments are removed during translation phase 3. This feature may look misleading, but it's indeed useful to create multi-line macros: #define MULTI_LINE_MACRO(x) if(x) { foo(); } else {} HTH, Ganesh [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Victor Bazarov Guest Posted: Thu Oct 06, 2005 1:38 pm    Post subject: Re: embedded '\' in comment bloodymir wrote: Quote: Recently I had a problem with an array initialization. After quite some testing I found the compiler (gcc3) was missing a line. I tracked down the problem to a "//" comment on the line before which ends on double backslash. It works like this: int main() { int i = 0; // \ ++i; cout << i; } So shall "++i" be part of the comment or not? Yes, it shall. Phase 2 of compilation removes the newline with immediately preceding backslash, so after phase 2 your code should look like this: int main() { int i = 0; // ++i; cout << i; } Phase 3 replaces each comment with a single space character. You could just add a space after the second backslash... V [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Jonathan Leffler Guest Posted: Thu Oct 06, 2005 1:48 pm    Post subject: Re: embedded '\' in comment bloodymir wrote: Quote: Hi! Recently I had a problem with an array initialization. After quite some testing I found the compiler (gcc3) was missing a line. I tracked down the problem to a "//" comment on the line before which ends on double backslash. It works like this: int main() { int i = 0; // \ ++i; cout << i; } So shall "++i" be part of the comment or not? backslash-newline combinations are removed in practically the first phase of pre-processing - so the comment is slash, slash, spaces, plus, plus, i, semi-colon. Put a space after the second backslash and you have one line of comment. / / silly comment! That's a comment too - comments are resolved after backslash-newline removal. / * This is * / That's a C style comment. Nasty, isn't it... -- Jonathan Leffler #include Email: [email]jleffler (AT) earthlink (DOT) net[/email], [email]jleffler (AT) us (DOT) ibm.com[/email] Guardian of DBD::Informix v2005.02 -- http://dbi.perl.org/ [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Martin Bonner Guest Posted: Thu Oct 06, 2005 3:10 pm    Post subject: Re: embedded '\' in comment bloodymir wrote: Quote: Hi! Recently I had a problem with an array initialization. After quite some testing I found the compiler (gcc3) was missing a line. I tracked down the problem to a "//" comment on the line before which ends on double backslash. It works like this: int main() { int i = 0; // \ ++i; cout << i; } So shall "++i" be part of the comment or not? The problem is not a DOUBLE backslash, it is a backslash at the very end of a line. Very early in the preprocessing (before #define is interpreted), multiple physical lines are merged into a single logical line by removing back-slash, end-of-line pairs. This is useful if you have a complex macro that you want to split over multiple lines. For example I have a macro to generate a forwarding function: #define DO_ACTION_NO_ARGS( Function ) HRESULT Dispatcher::Function() { HRESULT hResult = m_pTarget->Function( m_Arg1, m_Arg2 ); if ( FAILED( hResult ) ) { SetError( hResult, IDS_UNSUCCESSFUL, #Function ); } return hResult; } Line splicing is less useful in your example. Unfortunately, it happens BEFORE comments are removed. Thus the compiler sees: int main() { int i = 0; // ++i; cout << i; } It could be argued that line splicing should be applied AFTER comments have been removed, but it is about two decades too late to have that argument. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Yuri Khan Guest Posted: Thu Oct 06, 2005 3:11 pm    Post subject: Re: embedded '\' in comment bloodymir wrote: Quote: Hi! Recently I had a problem with an array initialization. After quite some testing I found the compiler (gcc3) was missing a line. I tracked down the problem to a "//" comment on the line before which ends on double backslash. It works like this: int main() { int i = 0; // \ ++i; cout << i; } So shall "++i" be part of the comment or not? On the second phase of translation (2.1/1), which is pretty early, all instances of backslash followed by newline are removed. This is called line splicing and widely used when defining multiline macros. The fact that the backslash is doubled is irrelevant. Thus, "++i;" shall be part of the comment. A good source editor with syntax highlighting will display it as such. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Tim Van Holder Guest Posted: Thu Oct 06, 2005 3:14 pm    Post subject: Re: embedded '\' in comment bloodymir wrote: Quote: Hi! Recently I had a problem with an array initialization. After quite some testing I found the compiler (gcc3) was missing a line. I tracked down the problem to a "//" comment on the line before which ends on double backslash. It works like this: int main() { int i = 0; // \ ++i; cout << i; } So shall "++i" be part of the comment or not? Yes it will. The standard (I don't have the current one handy, this is from the April 2005 draft, but I expect the wording to be similar) says, in section 2.1 (lex.phases): 2. Each instance of a new-line character and an immediately preceding backslash character is deleted, splicing physical source lines to form logical source lines. If, as a result, a character sequence that matches the syntax of a universal-character-name is produced, the behavior is undefined. If a source file that is not empty does not end in a new-line character, or ends in a new-line character immediately preceded by a backslash character, the behavior is undefined. There is nothing mentioned of any way of escaping that backslash, so ANY backslash-newline sequence is removed, including in your comment above. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze Guest Posted: Thu Oct 06, 2005 4:15 pm    Post subject: Re: embedded '\' in comment bloodymir wrote: Quote: Recently I had a problem with an array initialization. After quite some testing I found the compiler (gcc3) was missing a line. I tracked down the problem to a "//" comment on the line before which ends on double backslash. It works like this: int main() { int i = 0; // \ ++i; cout << i; } So shall "++i" be part of the comment or not? It's part of the comment, at least if there is no white space after the . See §2.1 Phases of translation, phase 2: "Each instance of a new-line character and an immediately preceding backslash character is deleted." This occurs before almost all other processing: only input character code mapping, replacement of trigraph sequences and possible generation of universal character names for characters outside the basic character set occurs earlier. (Note that trigraph substitution means that a line ending with ??/ will have the same effect.) The fact that a can "escape" an other , i.e. that '\' represents a single backslash, only occurs in phase 5, and then only in string and character literals. In practice, I can't imagine the case with a trigraph actually causing a problem; what meaningful comment could possibly end with ??/. On the other hand, I can well imagine a comment like: // Special treatment for Putting a blank behind the isn't a robust solution, either, since some systems or tools (or even some maintenance programmers) will remove trailing whitespace from lines in text files. The best solution I've found is that even in comments, character constants are in single quotes, e.g.: // Special treatment for '' -- 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 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Maciej Sobczak Guest Posted: Thu Oct 06, 2005 4:17 pm    Post subject: Re: embedded '\' in comment bloodymir wrote: Quote: int main() { int i = 0; // \ ++i; cout << i; } So shall "++i" be part of the comment or not? It is part of the comment, since line are spliced (on the newline immediately preceded by backslash) *before* comments are replaced by single space. -- Maciej Sobczak : http://www.msobczak.com/ Programming : http://www.msobczak.com/prog/ [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze Guest Posted: Fri Oct 07, 2005 1:09 pm    Post subject: Re: embedded '\' in comment Martin Bonner wrote: Quote: It could be argued that line splicing should be applied AFTER comments have been removed, but it is about two decades too late to have that argument. I think that if anyone were arguing for a change in the standard, it would be that line splicing only applies if there is an odd number of characters immediately preceding the new line (and that it then only removes the last character and the new line). Because most use of is in string or character constants, and in string and character constants, \ is an escape character sequence for , one rather aquires the intuition that escapes a following , so it is taken literally. All in all, however, I don't think that the problem is serious enough to be worth bothering with. The standards committee has more important things to do. -- 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 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top John Kewley Guest Posted: Tue Oct 18, 2005 9:32 pm    Post subject: Re: embedded '\' in comment kanze wrote: Quote: ... Putting a blank behind the isn't a robust solution, either, since some systems or tools (or even some maintenance programmers) will remove trailing whitespace from lines in text files. The best solution I've found is that even in comments, character constants are in single quotes, e.g.: // Special treatment for '' This works, although your comment [sic] about character constants could be misinterpreted since '\' would be the appropriate char constant. So "even in comments" is not quite right (if you are being pedantic). Quote: James Kanze GABI Software (the "other") JK [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze Guest Posted: Thu Oct 20, 2005 3:50 am    Post subject: Re: embedded '\' in comment John Kewley wrote: Quote: kanze wrote: ... Putting a blank behind the isn't a robust solution, either, since some systems or tools (or even some maintenance programmers) will remove trailing whitespace from lines in text files. The best solution I've found is that even in comments, character constants are in single quotes, e.g.: // Special treatment for '' This works, although your comment [sic] about character constants could be misinterpreted since '\' would be the appropriate char constant. So "even in comments" is not quite right (if you are being pedantic). It depends on whether the character constants in question are C++ character constants, or human character constants:-). If I'm really talking about C++ character constants, then I will use '\' in the comment. If I'm actually talking about specific character values, I'll write them as a human character constant. Actually, of course, I'm not nearly as rigorous about it as the above would imply. Comments are for human consumption, and human's are a lot better at parsing and understanding what I mean than any stupid compiler. On the other hand, I find that if you don't set off characters used as character values in some special way, it does cause problems for human readers. Depending on the context, I may use `x', "x", « x » (in France), or »x« or ,,x`` (in German), but on the whole, 'x' seems to be the easiest to type and the most appropriate when commenting C++ (or C or Java) code. Since so much of my code is C++, I've even extended this convention to shell scripts and makefiles. -- 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 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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