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 -- http://www.informatik.tu-darmstadt.de/TI/Mitarbeiter/cludwig.html LiDIA: http://www.informatik.tu-darmstadt.de/TI/LiDIA/Welcome.html