C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  superiority of unnamed namespaces over static linkage?          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message bert hubert Guest Posted: Mon Feb 16, 2004 11:10 pm    Post subject: superiority of unnamed namespaces over static linkage? I was reading the most excellent hardcover edition of the C++ Standard ( http://www.amazon.co.uk/exec/obidos/ASIN/0470846747 ) where I happened upon the claim that unnamed namespaces are superior over static linkage. Besides the fact that an unnamed namespace conveys more succinctly the local nature of a name, are there any other reaons for this superiority? In other words, are there semantical differences which are beneficial or efficiency reasons? Thanks. -- http://www.PowerDNS.com/pdns Try our new database driven nameserver! http://lartc.org Linux Advanced Routing & Traffic Control HOWTO [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Tue Feb 17, 2004 2:23 pm    Post subject: Re: superiority of unnamed namespaces over static linkage? In message <slrnc318bf.de7.ahu (AT) hubert (DOT) ds9a.nl>, bert hubert <ahu (AT) ds9a (DOT) nl> writes Quote: I was reading the most excellent hardcover edition of the C++ Standard ( http://www.amazon.co.uk/exec/obidos/ASIN/0470846747 ) where I happened upon the claim that unnamed namespaces are superior over static linkage. Besides the fact that an unnamed namespace conveys more succinctly the local nature of a name, are there any other reaons for this superiority? Things like classes cannot be declared static so if you want to hide them in a TU the unnamed namespace will cover it. Quote: In other words, are there semantical differences which are beneficial or efficiency reasons? Yes, things in the unnamed namespace have extern linkage and so can be used where something with internal linkage could not. Then there is the small issue that a named namespace can contain an unnamed namespace. -- Francis Glassborow ACCU Author of 'You Can Do It!' see http://www.spellen.org/youcandoit For project ideas and contributions: http://www.spellen.org/youcandoit/projects [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top bert hubert Guest Posted: Tue Feb 17, 2004 5:12 pm    Post subject: Re: superiority of unnamed namespaces over static linkage? On 2004-02-17, Francis Glassborow <francis (AT) robinton (DOT) demon.co.uk> wrote: Quote: Besides the fact that an unnamed namespace conveys more succinctly the local nature of a name, are there any other reaons for this superiority? Things like classes cannot be declared static so if you want to hide them in a TU the unnamed namespace will cover it. Ahh, I see. I had not previously realised that if there are two files both defining a class called 'Thing' differently that this will mess up things greatly at link time. Quote: Then there is the small issue that a named namespace can contain an unnamed namespace. To create an 'inner unnamed namespace' which does not affect the rest of the TU? Might be useful in include files containing real code I suspect (probably templates). Thanks for your elucidating answer! Bert. -- http://www.PowerDNS.com/ Open Source Database Driven Nameserver http://lartc.org Linux Advanced Routing & Traffic Control HOWTO [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Hendrik Schober Guest Posted: Wed Feb 18, 2004 1:39 am    Post subject: Re: superiority of unnamed namespaces over static linkage? bert hubert <ahu (AT) ds9a (DOT) nl> wrote: Quote: [...] Then there is the small issue that a named namespace can contain an unnamed namespace. To create an 'inner unnamed namespace' which does not affect the rest of the TU? Might be useful in include files containing real code I suspect (probably templates). I consider using unnamed namespaces in included files a bug. In each TU the files get included, all the symbols of these namespaces will be duplicated -- rarely ever what people want. Quote: Thanks for your elucidating answer! Bert. Schobi -- [email]SpamTrap (AT) gmx (DOT) de[/email] is never read I'm Schobi at suespammers dot org "Sometimes compilers are so much more reasonable than people." Scott Meyers [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Matthew Collett Guest Posted: Wed Feb 18, 2004 11:16 am    Post subject: Re: superiority of unnamed namespaces over static linkage? In article <slrnc34b57.eu9.ahu (AT) hubert (DOT) ds9a.nl>, bert hubert <ahu (AT) ds9a (DOT) nl> wrote: Quote: On 2004-02-17, Francis Glassborow <francis (AT) robinton (DOT) demon.co.uk> wrote: Then there is the small issue that a named namespace can contain an unnamed namespace. To create an 'inner unnamed namespace' which does not affect the rest of the TU? Might be useful in include files containing real code I suspect (probably templates). No, that is one thing it _isn't_ useful for. Unnamed namespaces should only be used in implementation files, not in include files, otherwise you are defining multiple copies of the things in the namespace. One reason to nest an unnamed namespace inside a named one is simply to get name look-up to work the way you want for the 'hidden' code. Best wishes, Matthew Collett -- Those who assert that the mathematical sciences have nothing to say about the good or the beautiful are mistaken. -- Aristotle [ 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 Feb 18, 2004 11:16 pm    Post subject: Re: superiority of unnamed namespaces over static linkage? bert hubert <ahu (AT) ds9a (DOT) nl> writes: Quote: I was reading the most excellent hardcover edition of the C++ Standard ( http://www.amazon.co.uk/exec/obidos/ASIN/0470846747 ) where I happened upon the claim that unnamed namespaces are superior over static linkage. Besides the fact that an unnamed namespace conveys more succinctly the local nature of a name, are there any other reaons for this superiority? In other words, are there semantical differences which are beneficial or efficiency reasons? You've got various answers. I'd point out that "supriority" here really depends on what you're doing. For example, the "static" keyword affects the second phase of name lookup (only function names with external linkage are considered). So, if you're using an exported template and you really mean your function not be "reachable"from another translation unit -- e.g. really really "local" -- then use "static". -- Gabriel Dos Reis [email]gdr (AT) integrable-solutions (DOT) net[/email] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Daveed Vandevoorde Guest Posted: Thu Feb 19, 2004 10:40 am    Post subject: Re: superiority of unnamed namespaces over static linkage? bert hubert <ahu (AT) ds9a (DOT) nl> wrote Quote: I was reading the most excellent hardcover edition of the C++ Standard ( http://www.amazon.co.uk/exec/obidos/ASIN/0470846747 ) where I happened upon the claim that unnamed namespaces are superior over static linkage. I would disagree that one is superior over the other. They're different ways of making things private to a translation unit. Quote: Besides the fact that an unnamed namespace conveys more succinctly the local nature of a name, are there any other reaons for this superiority? In other words, are there semantical differences which are beneficial or efficiency reasons? Declarations with internal linkage (i.e., "static") are invisible to phase-2 argument-dependent lookup (ADL). Declarations in unnamed namespaces have external linkage, which means they may be found through phase-2 ADL. However, since they have external linkage, they may potentially degrade the performance of the linking process. Daveed [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top bert hubert Guest Posted: Fri Feb 20, 2004 1:13 am    Post subject: Re: superiority of unnamed namespaces over static linkage? On 2004-02-18, Gabriel Dos Reis <gdr (AT) integrable-solutions (DOT) net> wrote: Quote: You've got various answers. I'd point out that "supriority" here really depends on what you're doing. For example, the "static" keyword affects the second phase of name lookup (only function names with external linkage are considered). So, if you're using an exported template and you really mean your function not be "reachable"from another translation unit -- e.g. really really "local" -- then use "static". Thanks to everybody. I now use unnamed namespaces for things that should be local, and I know why. -- http://www.PowerDNS.com/ Open Source Database Driven Nameserver http://lartc.org Linux Advanced Routing & Traffic Control HOWTO [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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