Re: [boost] [regex] Clearing failures in regex_regress on Tru64/CXX

From: Christoph Ludwig (cludwig_at_[hidden])
Date: 2005-06-29 08:47:40

On Wed, Jun 29, 2005 at 02:52:12PM +0200, Markus Schöpflin wrote:
> Christoph Ludwig wrote:
> 
> > On Wed, Jun 29, 2005 at 12:59:57PM +0100, John Maddock wrote:
> > 
> >>>Umm, now I'm confused. Why is explicit instantiation in more than one
> >>>translation unit illegal?
> >>
> >>I think I must be loosing it.... I can't find any such prohibition 
> >>anywhere.... I was so sure that I'd had problems with this in the past as 
> >>well!
> 
> > You are referring to the explicit instantiation of the same template
> > specialization in several TUs, aren't you?
> > 
> > That is indeed prohibited. I am too lazy to look for chapter and verse in my
> > copy of the standard but Vandevoorde and Josuttis write in "C++ Templates. The
> > Complete Guide":
> > 
> > Section 6.2.1:
> >   There should be, at most, one explicit instantiation of each distinct entity
> >   in a program. [...] Not following this rule usually results in linker errors
> >   that report duplicate definitions of the instantiated entities.
> 
> The problem is that the compiler _needs_ the explicit instantiation in 
> order not to end up with multiple defitions of a static variable used in 
> the class. And where should that instantiation be written, if not in the 
> header?

I don't know the peculiarities of the Tru64/CXX compiler. If it does not care
about multiple explicit instantiations (or even requires them for some
reason), then it may make sense to ignore the standard *on this particular
platform*. The explicit instantiation should be guarded by #ifdefs that filter
out all other platforms, though.

> > Section 10.5:
> >   The standard also specifies that there can be at most one explicit
> >   instantiation of a certain template specialization in a
> >   program. Furthermore, if a template specialization is explicitly
> >   instantiated, it should not be explicitly specialized, and vice versa.
> 
> I can't find anything in the standard (14.7) indicating this.

OK, I overcame my laziness :-) It is 14.7p5:

  No program shall explicitly instantiate any template more than once, both
  explicitly instantiate and explicitly specialize a template, or specialize a
  template more than once for a given set of template arguments. 
  An implementation is not required to diagnose a violation of this rule.

I admit I find Vandvoorde's and Josuttis' phrasing much easier to grok.

Regards

Christoph
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