C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Declaration or Definition          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message newmans Guest Posted: Sat Jun 26, 2004 2:23 am    Post subject: Declaration or Definition Perhaps one of the experts can straighten me out on this point... In Bjarne Stroustrup's book 'The C++ Programming Language' 3rd Edition, section 4.9, indicates that typedef complex<short> Point; is not only a declaration, but also a definition. The ISO/IEC 14882:2003(E) standard states in section 3.1 clause 2 that "A declaration is a definition unless it declares a function without specifying the function's body (8.4), it contains the extern specifier (7.1.1) or a linkage-specification24) (7.5) and neither an initializer nor a function-body, it declares a static data member in a class declaration(9.4), it is a class name declaration (9.1), or it is a typedef declaration (7.1.3), a using-declaration (7.3.3), or a using-directive (7.3.4)". So my question is , based on the (7.3.3) line of the clause, why is the typedef above a definition? Newman Back to top Phlip Guest Posted: Sat Jun 26, 2004 2:49 am    Post subject: Re: Declaration or Definition newmans wrote: Quote: Perhaps one of the experts can straighten me out on this point... In Bjarne Stroustrup's book 'The C++ Programming Language' 3rd Edition, section 4.9, indicates that typedef complex<short> Point; is not only a declaration, but also a definition. The ISO/IEC 14882:2003(E) standard states in section 3.1 clause 2 that "A declaration is a definition unless it declares a function without specifying the function's body (8.4), it contains the extern specifier (7.1.1) or a linkage-specification24) (7.5) and neither an initializer nor a function-body, it declares a static data member in a class declaration(9.4), it is a class name declaration (9.1), or it is a typedef declaration (7.1.3), a using-declaration (7.3.3), or a using-directive (7.3.4)". So my question is , based on the (7.3.3) line of the clause, why is the typedef above a definition? Because it instantiates a template into a class at that point. The new class bonds with identifiers seen at its location. Using Point later doesn't re-bond these identifiers, as #define Point complex<short> would. This is why one should always instantiate templates with typedefs, to control their definition point. -- Phlip http://industrialxp.org/community/bin/view/Main/TestFirstUserInterfaces Back to top Rob Williscroft Guest Posted: Sat Jun 26, 2004 7:32 am    Post subject: Re: Declaration or Definition Phlip wrote in news:dv5Dc.1291$e71.351 (AT) newssvr33 (DOT) news.prodigy.com in comp.lang.c++: Quote: newmans wrote: Perhaps one of the experts can straighten me out on this point... In Bjarne Stroustrup's book 'The C++ Programming Language' 3rd Edition, section 4.9, indicates that typedef complex<short> Point; is not only a declaration, but also a definition. The ISO/IEC 14882:2003(E) standard states in section 3.1 clause 2 that "A declaration is a definition unless [...] or it is a typedef declaration (7.1.3), [...] So my question is , based on the (7.3.3) line of the clause, why is the typedef above a definition? The Standard says it isn't, nothing else matters. Quote: Because it instantiates a template into a class at that point. There is no class-template being instantiated here. Quote: The new class bonds with identifiers seen at its location. Using Point later doesn't re-bond these identifiers, as #define Point complex<short would. This is why one should always instantiate templates with typedefs, to control their definition point. This was true with some pre-standard version's of C++. With Standard C++, the problem is /mostly/ mitigated with 2-phase name lookup, if that isn't good enough you can use: template complex< short >; But it only works if *all* of complex<>'s member's are instantiatable with the 'short' argument. Rob. -- http://www.victim-prime.dsl.pipex.com/ Back to top Chris Gordon-Smith Guest Posted: Sat Jun 26, 2004 4:52 pm    Post subject: Re: Declaration or Definition newmans wrote: Quote: Perhaps one of the experts can straighten me out on this point... In Bjarne Stroustrup's book 'The C++ Programming Language' 3rd Edition, section 4.9, indicates that typedef complex<short> Point; is not only a declaration, but also a definition. The ISO/IEC 14882:2003(E) standard states in section 3.1 clause 2 that "A declaration is a definition unless it declares a function without specifying the function's body (8.4), it contains the extern specifier (7.1.1) or a linkage-specification24) (7.5) and neither an initializer nor a function-body, it declares a static data member in a class declaration(9.4), it is a class name declaration (9.1), or it is a typedef declaration (7.1.3), a using-declaration (7.3.3), or a using-directive (7.3.4)". So my question is , based on the (7.3.3) line of the clause, why is the typedef above a definition? Interesting question. Here is my understanding of the situation. Section 4.9 of Stroustrup's book says "definitions [...] also define an entity for the name to which they refer. ... For Point it is the type complex<short> so that point becoms a synonym for complex<short>". Therefore, the definition part of this typedef is that it is defining a new type (the 'entity'), complex<short>. The type complex<short> would not exist without the typedef. I assume that the line of the standard you intended to mention was 7.1.3. This says that a typedef declaration is not a definition. I would think that an example of this would be typedef int int32; This is not defining an 'entity'. In this case, the entity is the built in type int, which already exists. Therefore this typedef is just declaring the name int32 as a synonym for the already existing type 'int'. -- Chris Gordon-Smith London Homepage: http://graffiti.virgin.net/c.gordon-smith/ Email Address: Please see my Home Page Back to top Alf P. Steinbach Guest Posted: Sat Jun 26, 2004 6:14 pm    Post subject: Re: Declaration or Definition * newmans: Quote: Perhaps one of the experts can straighten me out on this point... In Bjarne Stroustrup's book 'The C++ Programming Language' 3rd Edition, section 4.9, indicates that typedef complex<short> Point; is not only a declaration, but also a definition. The ISO/IEC 14882:2003(E) standard states in section 3.1 clause 2 that "A declaration is a definition unless it declares a function without specifying the function's body (8.4), it contains the extern specifier (7.1.1) or a linkage-specification24) (7.5) and neither an initializer nor a function-body, it declares a static data member in a class declaration(9.4), it is a class name declaration (9.1), or it is a typedef declaration (7.1.3), a using-declaration (7.3.3), or a using-directive (7.3.4)". So my question is , based on the (7.3.3) line of the clause, why is the typedef above a definition? It isn't. How exactly does Bjarne, in your opinion, "indicate" that it is? -- 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? Back to top Russell Hanneken Guest Posted: Sat Jun 26, 2004 6:53 pm    Post subject: Re: Declaration or Definition Alf P. Steinbach wrote: Quote: * newmans: In Bjarne Stroustrup's book 'The C++ Programming Language' 3rd Edition, section 4.9, indicates that typedef complex<short> Point; is not only a declaration, but also a definition. How exactly does Bjarne, in your opinion, "indicate" that it is? After giving a list of declarations, including typedef complex<short> Point; Stroustrup writes: Most of these declarations are also definitions, that is, they also define an entity for the name to which they refer. For ch, that entity is the appropriate amount of memory to be used as a variable. . . . For day, it is the specified function. . . . For Point, it is the type complex<short> so that Point becomes a synonym for complex<short>. Of the declarations above, only double sqrt(double); extern int error_number; struct User; are not also definitions; that is, the entity they refer to must be defined elsewhere. Seems pretty clear to me. I'm quoting from the 3rd printing of the "special edition." I checked Stroustrup's errata pages to see if he corrected himself in a later printing, but I don't see any mention of this issue. -- Russell Hanneken [email]eunaarxra (AT) cbobk (DOT) pbz[/email] Use ROT13 to decode my email address. Back to top Russell Hanneken Guest Posted: Sat Jun 26, 2004 7:01 pm    Post subject: Re: Declaration or Definition Chris Gordon-Smith wrote: Quote: The type complex<short> would not exist without the typedef. I assume that the line of the standard you intended to mention was 7.1.3. This says that a typedef declaration is not a definition. I would think that an example of this would be typedef int int32; This is not defining an 'entity'. In this case, the entity is the built in type int, which already exists. Therefore this typedef is just declaring the name int32 as a synonym for the already existing type 'int'. Interesting distinction, but unfortunately the standard says "A declaration is a definition unless . . . it is a typedef declaration." For your argument to work, you would have to say that typedef complex<short> Point; is a definition, but not a typedef declaration. I don't think the standard means to say that. -- Russell Hanneken [email]eunaarxra (AT) cbobk (DOT) pbz[/email] Use ROT13 to decode my email address. Back to top Alf P. Steinbach Guest Posted: Sat Jun 26, 2004 7:40 pm    Post subject: Re: Declaration or Definition * Russell Hanneken: Quote: Alf P. Steinbach wrote: * newmans: In Bjarne Stroustrup's book 'The C++ Programming Language' 3rd Edition, section 4.9, indicates that typedef complex<short> Point; is not only a declaration, but also a definition. How exactly does Bjarne, in your opinion, "indicate" that it is? After giving a list of declarations, including typedef complex<short> Point; Stroustrup writes: Most of these declarations are also definitions, that is, they also define an entity for the name to which they refer. For ch, that entity is the appropriate amount of memory to be used as a variable. . . . For day, it is the specified function. . . . For Point, it is the type complex<short> so that Point becomes a synonym for complex<short>. Of the declarations above, only double sqrt(double); extern int error_number; struct User; are not also definitions; that is, the entity they refer to must be defined elsewhere. Seems pretty clear to me. I'm quoting from the 3rd printing of the "special edition." I checked Stroustrup's errata pages to see if he corrected himself in a later printing, but I don't see any mention of this issue. Urm, well. The only practical difference seems to be that while you cannot have multiple definitions of the same name (except internal linkage in different compilation units), you can have multiple typedef's of the same name provided they denote the same type. I'm forwarding this to Bjarne, assuming his spam-filter doesn't automatically trash mails from me... ;-) -- 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? Back to top Chris Gordon-Smith Guest Posted: Sat Jun 26, 2004 11:49 pm    Post subject: Re: Declaration or Definition Russell Hanneken wrote: Quote: Chris Gordon-Smith wrote: The type complex<short> would not exist without the typedef. I assume that the line of the standard you intended to mention was 7.1.3. This says that a typedef declaration is not a definition. I would think that an example of this would be typedef int int32; This is not defining an 'entity'. In this case, the entity is the built in type int, which already exists. Therefore this typedef is just declaring the name int32 as a synonym for the already existing type 'int'. Interesting distinction, but unfortunately the standard says "A declaration is a definition unless . . . it is a typedef declaration." For your argument to work, you would have to say that typedef complex<short> Point; is a definition, but not a typedef declaration. I don't think the standard means to say that. Fair enough. But that does seem to leave us in a rather contradictory situation: 1) typedef int int32; // Declaration that is not a definition 2) typedef complex<short> Point; // Declaration that is also a definition (as stated by Stroustrup section 4.9) 3) The standard says "A declaration is a definition unless . . . it is a typedef declaration." (2) and (3) can only be consistent if (2) is a declaration (and a definition), but not a 'typedef declaration'. Any ideas on a way out of this contradictory situation? What is a 'typedef declaration'? -- Chris Gordon-Smith London Homepage: http://graffiti.virgin.net/c.gordon-smith/ Email Address: Please see my Home Page 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.