This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-12-19

46. Explicit instantiation of member templates

Section: 13.9.3  [temp.explicit]     Status: NAD     Submitter: John H. Spicer     Date: 28 Jan 1998

Is the second explicit instantiation below well-formed?

    template <class T> struct A {
        template <class T2> void f(T2){}
    };

    template void A<int>::f(char); // okay

    template template void A<int>::f(float); // ?
Since multiple "template<>" clauses are permitted in an explicit specialization, it might follow that multiple "template" keywords should also be permitted in an explicit instantiation. Are multiple "template" keywords not allowed in an explicit instantiation? The grammar permits it, but the grammar permits lots of stuff far weirder than that. My opinion is that, in the absence of explicit wording permitting that kind of usage (as is present for explicit specializations) that such usage is not permitted for explicit instantiations.

Rationale (04/99): The Standard does not describe the meaining of multiple template keywords in this context, so the example should be considered as resulting in undefined behavior according to Clause 3 [intro.defs] “undefined behavior.”