C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Splicing/Concatenation and Undefined Behavior          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message Greg Hickman Guest Posted: Fri Jul 02, 2004 5:40 am    Post subject: Splicing/Concatenation and Undefined Behavior Why do 2.1(2) and 2.1(4) say that undefined behavior occurs if a character sequence that matches the syntax of a universal character name results from splicing physical source lines or token concatenation? Does this mean it's possible to construct a well-formed program that unwittingly contains such undefined behavior? If so, what might it look like and what can we do to prevent it? Thanks, Greg --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top James Kuyper Guest Posted: Fri Jul 02, 2004 7:47 pm    Post subject: Re: Splicing/Concatenation and Undefined Behavior Greg Hickman <greg.hickman (AT) lmco (DOT) com> wrote Quote: Why do 2.1(2) and 2.1(4) say that undefined behavior occurs if a character sequence that matches the syntax of a universal character name results from splicing physical source lines or token concatenation? Does this mean it's possible to construct a well-formed program that unwittingly contains such undefined behavior? If so, what might it look like and what can we do to prevent it? // Splice occurs as described in 2.1p2: int helloU03 88 = 0; // Splice occurs as described in 2.1p4: #define STR(a,b) a##b #define STRING(a,b) STR(a,b) int STRING(worldU03,89) = 1; Note: in contrast, the following code would be no problem: int helloU0388 = 0; int worldU0399 = 1; To avoid the problem, just be careful about using string catenation, and escaped new-lines. --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top Greg Hickman Guest Posted: Fri Jul 02, 2004 8:46 pm    Post subject: Re: Splicing/Concatenation and Undefined Behavior "James Kuyper" <kuyper (AT) wizard (DOT) net> wrote Quote: // Splice occurs as described in 2.1p2: int helloU03 88 = 0; // Splice occurs as described in 2.1p4: #define STR(a,b) a##b #define STRING(a,b) STR(a,b) int STRING(worldU03,89) = 1; Note: in contrast, the following code would be no problem: int helloU0388 = 0; int worldU0399 = 1; To avoid the problem, just be careful about using string catenation, and escaped new-lines. I thought these might be the kinds of scenarios being described in the standard, but wanted to be sure. It isn't readily apparent to me why they can lead to undefined behavior. Thanks, Greg --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top James Kuyper Guest Posted: Sun Jul 04, 2004 7:22 am    Post subject: Re: Splicing/Concatenation and Undefined Behavior [email]greg.hickman (AT) lmco (DOT) com[/email] (Greg Hickman) wrote in message news:<cc4enu$lcq3 (AT) cui1 (DOT) lmms.lmco.com>... Quote: "James Kuyper" <kuyper (AT) wizard (DOT) net> wrote in message news:8b42afac.0407020400.300d52d8 (AT) posting (DOT) google.com... // Splice occurs as described in 2.1p2: int helloU03 88 = 0; // Splice occurs as described in 2.1p4: #define STR(a,b) a##b #define STRING(a,b) STR(a,b) int STRING(worldU03,89) = 1; Note: in contrast, the following code would be no problem: int helloU0388 = 0; int worldU0399 = 1; To avoid the problem, just be careful about using string catenation, and escaped new-lines. I thought these might be the kinds of scenarios being described in the standard, but wanted to be sure. It isn't readily apparent to me why they can lead to undefined behavior. Giving such splices undefined behavior, relieves the implementation of any responsibility for checking the results of splices for whether or not they contain UCNs. On an implementation which takes advantage of that fact, the most likely form of undefined behavior would be a failure to notice that the spliced-together identifier should have been identified as a match with one that never needed splicing. Thus, it wouldn't recognize the two following lines as containing the same identifier: int helloU0388; helloU03 88 = 1; --- [ 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.jamesd.demon.co.uk/csc/faq.html ] Back to top Ben Hutchings Guest Posted: Mon Jul 05, 2004 10:03 pm    Post subject: Re: Splicing/Concatenation and Undefined Behavior Greg Hickman wrote: Quote: Why do 2.1(2) and 2.1(4) say that undefined behavior occurs if a character sequence that matches the syntax of a universal character name results from splicing physical source lines or token concatenation? The justification I see is that this allows implementations to interpret universal-character-names at whatever stage is most convenient. This may not be the actual reason for the decision. Quote: Does this mean it's possible to construct a well-formed program that unwittingly contains such undefined behavior? If so, what might it look like and what can we do to prevent it? It is possible to contrive a program with undefined behaviour due to 2.1(2): int u 0100 = 0; As for 2.1(4), I must admit I have found it useful to construct UCNs by token concatenation in a macro in order to work with an implementation that does not support UCNs (VC++ 6), for which the macro was defined differently. If you only need to support more standard compilers then I do not see that this would be necessary. I think it should only take a little self-disicpline to ensure that whenever you type u you also type a full 4 digits after it. A search through your source files with the (Perl-style) regexp: (^|[^\])(\\)*\(\nu|u.{0,3}[^0-9A-Fa-f]) should reveal any deviation from this rule, unless you use trigraphs, in which case you would need a slightly more complex but rather longer regex. --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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