C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  typedef linkage          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Saul Guest Posted: Thu Sep 11, 2003 3:34 pm    Post subject: typedef linkage Hi, I've stumbled across the following line in C++PL (special edition): ----- 9.2 (last line in page 199) - By default, consts and typedefs have internal linkage. ----- What does linkage has to do with typedefs? Can a typedef be external? Thanks, Saul [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Siemel Naran Guest Posted: Fri Sep 12, 2003 10:27 am    Post subject: Re: typedef linkage "Saul" <stamari (AT) bigfoot (DOT) com> wrote in message Quote: I've stumbled across the following line in C++PL (special edition): ----- 9.2 (last line in page 199) - By default, consts and typedefs have internal linkage. ----- What does linkage has to do with typedefs? Can a typedef be external? I think it means this ==> // a.h // b.h // a.cpp #include "a.h" typedef int Junk; namespace { void f() { Junk j; } } // b.cpp #include "b.h" typedef double Junk; namespace { void f() { Junk j; } } // main.cpp #include "a.h" #include "b.h" int main() { } The above program should link. If typedefs had external linkage, then the program would fail to link because a.obj and b.obj both define a symbol Junk. -- +++++++++++ Siemel Naran [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze@gabi-soft.fr Guest Posted: Fri Sep 12, 2003 10:30 am    Post subject: Re: typedef linkage [email]stamari (AT) bigfoot (DOT) com[/email] (Saul) wrote in message news:<62ce112c.0309110053.16ac3562 (AT) posting (DOT) google.com>... Quote: I've stumbled across the following line in C++PL (special edition): ----- 9.2 (last line in page 199) - By default, consts and typedefs have internal linkage. ----- What does linkage has to do with typedefs? All names have linkage. A typedef defines a name. The name it defines must have some sort of linkage. Quote: Can a typedef be external? No. §7.1.1/5: "The extern specifier can be applied only to names of objects and functions and to anonymous unions." The name defined by a typedef doesn't qualify. On the other hand, trying to apply the text in §3.5 to typedef's leads to some contradictions -- but I think it is a problem in the wording of §3.5 (or my understanding of it). Basically, §3.5/2 defines the three types of linkage with regards to the "entity it [the name] denotes". In the case of a typedef, this would be the actual type. I don't think that this is what was meant -- it shouldn't be a question of being able to refer to the entity "by names from other scopes", but to refer to it "by the same name from other scopes". (But even that is ambiguous -- if I write "typedef int x", in two different scopes, I can refer to the type int using the name x in both scopes.) At any rate, I'm fairly sure that the intention is for names defined by a typedef to have internal linkage if the typedef is at namespace scope, and no linkage if the typedef is at block scope. Although I'm not quite sure how to find this in the standard. -- James Kanze GABI Software mailto:kanze (AT) gabi-soft (DOT) fr Conseils en informatique orientée objet/ http://www.gabi-soft.fr Beratung in objektorientierter Datenverarbeitung 11 rue de Rambouillet, 78460 Chevreuse, France, +33 (0)1 30 23 45 16 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Saul Guest Posted: Sat Sep 13, 2003 8:17 am    Post subject: Re: typedef linkage "Siemel Naran" wrote in message > [snip] Quote: The above program should link. If typedefs had external linkage, then the program would fail to link because a.obj and b.obj both define a symbol Junk. This program should indeed link but not because of the internal typedefs linkage but because typedefs are type aliases and are not true types. In the example you gave no type 'Junk' is actually created by the compiler, The type 'int' is used wherever 'Junk' is mentioned. Saul [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gabriel Dos Reis Guest Posted: Wed Sep 17, 2003 7:58 pm    Post subject: Re: typedef linkage [email]kanze (AT) gabi-soft (DOT) fr[/email] writes: Quote: stamari (AT) bigfoot (DOT) com (Saul) wrote in message news:<62ce112c.0309110053.16ac3562 (AT) posting (DOT) google.com>... I've stumbled across the following line in C++PL (special edition): ----- 9.2 (last line in page 199) - By default, consts and typedefs have internal linkage. ----- What does linkage has to do with typedefs? All names have linkage. That statement is confusing. Some names no linkage. See 3.5/2, in particular -- when a name has /no linkage/, the entity it denotes cannot be referred to by names from other scopes. You may consider "no linkage" as a kind of linkage, but then you'd have to reconcile that reading with A name is said to have /linkage/ when it might denote the same object, reference, function, type, template, namespace or value as a name introduced by a declaration in another scope: which strongly suggest that a name cannot have linkage, and this possibility is in better adequacy with the first quote. You also have to take into account The name of a function declared in block scope, and the name of an object declared by a block scope extern declaration, have linkage. which also strongly suggests that the names of some entities do not have linkage. [...] Quote: I don't think that this is what was meant -- it shouldn't be a question of being able to refer to the entity "by names from other scopes", but to refer to it "by the same name from other scopes". I can come to agree with that. Quote: At any rate, I'm fairly sure that the intention is for names defined by a typedef to have internal linkage if the typedef is at namespace scope, that would have to be weighted against namespace N { typedef struct { int bar; } foo; } where the typedef-name N::foo has an external linkage. Quote: and no linkage if the typedef is at block scope. Although I'm not quite sure how to find this in the standard. A typedef-name being syntactically equivalent to a keyword, I would think that a typedef itself do not have a linkage, except when it designates an unnamed class. At least that would be consistent with 3.5/8 [...] If a declaration uses a typedef name, it is the linkage of the type name to which the typedef refers that is considered. -- Gabriel Dos Reis, [email]dosreis (AT) cmla (DOT) ens-cachan.fr[/email] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze@gabi-soft.fr Guest Posted: Fri Sep 19, 2003 10:29 am    Post subject: Re: typedef linkage Gabriel Dos Reis <dosreis (AT) cmla (DOT) ens-cachan.fr> wrote Quote: kanze (AT) gabi-soft (DOT) fr writes: | [email]stamari (AT) bigfoot (DOT) com[/email] (Saul) wrote in message | news:<62ce112c.0309110053.16ac3562 (AT) posting (DOT) google.com>... | > I've stumbled across the following line in C++PL (special edition): | > ----- | > 9.2 (last line in page 199) - | > By default, consts and typedefs have internal linkage. | > ----- | > What does linkage has to do with typedefs? | All names have linkage. That statement is confusing. Isn't it, though. Perhaps "all names have a type of linkage"? Anyhow, the point I was really trying to make was that linkage is associated with names; in the sense that a name has in some way a linkage attribute (which in data base terms, might be null). Quote: [...] | At any rate, I'm fairly sure that the intention is for names defined by | a typedef to have internal linkage if the typedef is at namespace scope, that would have to be weighted against namespace N { typedef struct { int bar; } foo; } where the typedef-name N::foo has an external linkage. Good point. I'd forgotten about this C compatibility hack (which basically results in the typedef'ed name being treated as if it has followed the keyword struct, without the typedef). Quote: | and no linkage if the typedef is at block scope. Although I'm not | quite sure how to find this in the standard. A typedef-name being syntactically equivalent to a keyword, I would think that a typedef itself do not have a linkage, except when it designates an unnamed class. At least that would be consistent with 3.5/8 [...] If a declaration uses a typedef name, it is the linkage of the type name to which the typedef refers that is considered. Good point. And when the typedef name is used elsewhere? I think it is always "as if" the actual type had been used. If so, then it would make sense to say that the typedef'ed name has no linkage. -- James Kanze GABI Software mailto:kanze (AT) gabi-soft (DOT) fr Conseils en informatique orientée objet/ http://www.gabi-soft.fr Beratung in objektorientierter Datenverarbeitung 11 rue de Rambouillet, 78460 Chevreuse, France, +33 (0)1 30 23 45 16 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gabriel Dos Reis Guest Posted: Fri Sep 19, 2003 11:19 am    Post subject: Re: typedef linkage [email]kanze (AT) gabi-soft (DOT) fr[/email] writes: [...] Quote: | and no linkage if the typedef is at block scope. Although I'm not | quite sure how to find this in the standard. A typedef-name being syntactically equivalent to a keyword, I would think that a typedef itself do not have a linkage, except when it designates an unnamed class. At least that would be consistent with 3.5/8 [...] If a declaration uses a typedef name, it is the linkage of the type name to which the typedef refers that is considered. Good point. And when the typedef name is used elsewhere? I think it is always "as if" the actual type had been used. yes, that essentially is what I understand by "a typedef-name is syntactically equivalent to a keyword and names the type associated with the identifier" It is kind of "scoped macro" -- yes, the analogy is poor :-/ Quote: If so, then it would make sense to say that the typedef'ed name has no linkage. that would be my inclination. -- Gabriel Dos Reis <gdr (AT) integrable-solutions (DOT) net> [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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