c++ - extern template & incomplete types -

recently when trying optimize include hierarchy stumbled upon file a.hpp:

template<class t> class {   using t = typename t::a_t; };  class b;  extern template class a<b>; 

which seems ill-formed. in fact seems if extern template statement @ end causes instantiation of a<b> causes compiler complain incomplete type.

my goal have been define a<b> in a.cpp:

#include <b.hpp> template class a<b>; 

this way avoid having include b.hpp a.hpp seems idea reduce compile time. not work (a.hpp on doesn't compile!) there better way of doing this?

note: of course not use explicit template instantiation not want! "precompile" a<b> save compilation time if used, if a<b> not used don't want include b.hpp in every file uses a.hpp!

from http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2003/n1448.pdf

the extern specifier used declare explicit instantiation of class template suppresses explicit instantiations of definitions of member functions , static data members not specialized in translation unit containing declaration.

thus, if definition of b required in a, cannot use extern template without knowledge of b. of course try rid of requirement. in given case, remove using t declaration , have meta function yield type:

template<typename t> struct get_a_t;  template<typename t> struct get_a_t<a<t>> {    using type = typename t::a_t; }; 

not sure feasible in case. needs store b or b::a_t, need b. references , pointers ok, though.