Re: [boost] [optional]none.hpp causes VC7.1 precompiled headertobehave erratically

From: Joe Gottman (jgottman_at_[hidden])
Date: 2006-07-17 20:01:57

"Fernando Cacciola" <fernando_cacciola_at_[hidden]> wrote in message 
news:e9h475$3oq$1_at_sea.gmane.org...
> Gennaro Prota wrote:
>> On Sun, 16 Jul 2006 01:16:10 -0400, "Sean Huang"
>> <huangsean_at_[hidden]> wrote:
>>
>>> Interestingly, I notice that boost::none is defined in an unnamed
>>> namespace.
>>
>> That really surprised me. As a rule of thumb unnamed namespaces
>> shouldn't be used in headers. In the rare cases where a good reason
>> exists for doing the contrary, that should be adequately commented.
>>
>>> Since boost::none is const and thus has internal linkage so I
>>> removed the unnamed namespace and everything started working again.
>>> So my questions is what purpose the unnamed namespace serves? Can it
>>> be removed in light of this problem with precompiled headers?
>>
>> If there are good reasons for having it, it should be removed
>> regardless of the precompiled header problem. An unnamed namespace has
>> a name which is unique to its translation unit, not source file.
>>
> OK. To be honest I can't find myself any good reason for that.
> I have a faint recollection that I just duplicated the implementation of
> some other boost library which is, or was, putting such "global constants"
> on the unnamed namespace, but I can't find it now.
> In any event, I agree that the unnamed namespace should be removed, so
> unless someone comes up with a good reason for that in the next few days,
> I'll just proceed with it.
>
>
>> I'm also surprised that both none.hpp and none_t.hpp are in the boost
>> root dir. Do they have any general usage that justifies that? (I think
>> I was guilty of a similar sin with non_type.hpp and I'd be happy to
>> move it)
>
> The truth is that I see "none" as a general token and not something 
> specific
> to optional.
> You could use it in variant, any, tuple, etc... so like most stuff in 
> boost
> is there so others can use it... but of course is up to those others to go
> ahead and do it.

   Just out of curiosity, why didn't you declare none_t as an enum?  If you 
define
       enum none_t {none = 0};

then you have easily defined the value boost::none in a header file so that 
it can be included in as many headers as possible without bumping into the 
ODR.

Joe Gottman