Extern template with variadic arguments doesn't compile
c++, c++11, templates, variadic-templates
Solution
The `extern` keyword does something different than you expect - if I understand correctly what you expect, of course.
The `extern` keyword is applied to explicit instantiations of a template, and it prevents the compiler from generating implicitly the code for that template while processing a certain translation unit. Per Paragraph 14.7.2/2 of the C++11 Standard:
There are two forms of explicit instantiation: an explicit instantiation definition and an explicit instantiation declaration. An explicit instantiation declaration begins with the `extern` keyword.
Without the `extern` keyword, the compiler would generate code for (say) `log(double, int)` in each translation unit that contains calls to `log(double, int)`, and this code - which would and should be identical for all translation units - would be eventually merged by the linker (the linker would basically discard all duplicates and keep only one).
The `extern` keyword saves you from this waste of compilation time by telling the compiler: "Trust me, somebody else will instantiate this template somewhere else - you don't need to do it now". But that promise must be fulfilled.
So for instance, if you have this primary template:
template<typename... Xs> void log(Xs... xs);
And you declare this explicit instantiation:
extern template void log(int, double);
Than you must have a corresponding explicit instantiation in some translation unit:
template void log(int, double)
Otherwise, the compiler will never ever produce code for `log<int, double>(int, double)`, and the linker will complain about undefined references.
Problem
I try to create a extern template with variadic arguments like: ``` extern template<typename... XS> void log( XS... xs ); ``` But gcc 7.2 doesn't compile it, and show the error: ``` error: expected unqualified-id before ‘<’ token ``` I check the gcc status in c++11, and extern templates should work, isn't it?