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Do all functions default to external linkage by default?


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#1 noatom   Members   -  Reputation: 785

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Posted 05 May 2013 - 04:19 PM

I just have a hard time thinking about some functions that default to internal linkage,are there any? by default of course.



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#2 Brother Bob   Moderators   -  Reputation: 8626

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Posted 05 May 2013 - 04:31 PM

Inline or static functions have internal linkage, non-inline and non-static functions, which I presume you mean by "default" since you don't decorate them with additional keywords, have external linkage.



#3 noatom   Members   -  Reputation: 785

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Posted 05 May 2013 - 05:49 PM

thanks a lot,that clarifies everything.



#4 Paradigm Shifter   Crossbones+   -  Reputation: 5440

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Posted 05 May 2013 - 06:52 PM

Functions (and everything else) in an anonymous namespace have internal linkage as well, since that has deprecated the use of the static qualifier in C++ now.


Edited by Paradigm Shifter, 05 May 2013 - 06:52 PM.

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#5 Cornstalks   Crossbones+   -  Reputation: 6991

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Posted 05 May 2013 - 07:27 PM


Functions (and everything else) in an anonymous namespace have internal linkage as well, since that has deprecated the use of the static qualifier in C++ now.


Note that C++11 un-deprecated the usage of the static qualifier.
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#6 Paradigm Shifter   Crossbones+   -  Reputation: 5440

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Posted 05 May 2013 - 07:36 PM

Not that making it deprecated stopped people using it anyway ;)

 

So why did they do that? I suppose it is easier to tell from the declaration something has internal linkage if it has a static qualifier rather than having to check to see if it is inside an anonymous namespace...


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#7 Cornstalks   Crossbones+   -  Reputation: 6991

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Posted 05 May 2013 - 08:00 PM


So why did they do that? I suppose it is easier to tell from the declaration something has internal linkage if it has a static qualifier rather than having to check to see if it is inside an anonymous namespace...


They just changed the wording so that it's not longer deprecated. The C++ committee likely still thinks anonymous namespaces are superior, but (from what I've read) decided it's pointless to officially deprecate it. There was no technical need for it to be deprecated.
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#8 Brother Bob   Moderators   -  Reputation: 8626

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Posted 06 May 2013 - 12:22 AM

Functions (and everything else) in an anonymous namespace have internal linkage as well, since that has deprecated the use of the static qualifier in C++ now.

I know I've responded to you like this before so it kind of feels like I'm picking on you, but... symbols in an anonymous namespace typically have external linkage, they just happen to have a unique name automatically associated with them that is not accessible from the outside.

 

Wouldn't say it has much of an impact in every-day code, but one place where is matters is for example template parameters: reference of pointer template parameters can only be instantiated with symbols of external linkage. Therefore, you can pass the reference or pointer to a variable within an anonymous namespace, but not a static variable, as a reference or pointer template parameter.

static int bar1 = 0;
namespace { int bar2 = 0; }
 
template<int *> void foo() {}
 
int main() {
    foo<&bar1>(); // error, bar1 does not have external linkage
    foo<&bar2>(); // ok, bar2 has external linkage
}


#9 noatom   Members   -  Reputation: 785

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Posted 06 May 2013 - 03:53 AM

Not really following you guys.So,if I put a function/variable in a namespace created by me,the function/variable will have internal linkage right?



#10 Brother Bob   Moderators   -  Reputation: 8626

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Posted 06 May 2013 - 05:10 AM

Unless you make it static, a function or variable in a namespace typically has external linkage. And that applies to anonymous namespaces as well.



#11 SiCrane   Moderators   -  Reputation: 9674

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Posted 06 May 2013 - 06:14 AM

I know I've responded to you like this before so it kind of feels like I'm picking on you, but... symbols in an anonymous namespace typically have external linkage, they just happen to have a unique name automatically associated with them that is not accessible from the outside.

This is actually different in C++03 and C++11. In C++03 they have external linkage, but C++11 changed names in anonymous namespaces to have internal linkage. This was accompanied by a change to the template argument rules so that it's legal to use an object with internal linkage as a template argument. The relevant sections of the standards to compare are 3.5 for program linkage rules and 14.3.2 for the non-type template argument rules (same section numbers in both).

#12 Brother Bob   Moderators   -  Reputation: 8626

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Posted 06 May 2013 - 06:46 AM

I see that in section 14 about internal linkage, and VS2012 appears not to be compliant on that part. But which paragraph in 3.5 refers to unnamed namespaces having internal linkage? I can, now that I read it again, assume that it's paragraph 4 and that it has an ambiguous formulation because, as I read it, it refers to nested namespaces inside an unnamed namespace.



#13 Álvaro   Crossbones+   -  Reputation: 13933

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Posted 06 May 2013 - 07:15 AM

I see that in section 14 about internal linkage, and VS2012 appears not to be compliant on that part. But which paragraph in 3.5 refers to unnamed namespaces having internal linkage? I can, now that I read it again, assume that it's paragraph 4 and that it has an ambiguous formulation because, as I read it, it refers to nested namespaces inside an unnamed namespace.

An unnamed namespace or a namespace declared directly or indirectly within an unnamed namespace has
internal linkage.

How is that ambiguous? It refers to both.

#14 Brother Bob   Moderators   -  Reputation: 8626

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Posted 06 May 2013 - 07:52 AM

Using parentheses to group the OR: "(An unnamed namespace or a named namespace) declared directly or indirectly within an unnamed namespace..." as opposed to "An unnamed namespace or (a named namespace declared directly or indirectly within an unnamed namespace)..." I read the first, but apparently the intended meaning is the second.



#15 Álvaro   Crossbones+   -  Reputation: 13933

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Posted 06 May 2013 - 11:42 AM

Oh, I see. I guess this sentence is just very poorly written: I also object to the use of "OR" instead of "AND". To me `A or B has internal linkage' means `A has internal linkage' OR `B has internal linkage', but what they are trying to say is `A has internal linkage' AND `B has internal linkage', which I would write as `A and B have internal linkage'.




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