There is a template class with three of member functions without implementation. But at the end of the header file of the class, something like the following appears:

extern template void template_class::member_function();

When compiling and linking to generate this library, error lnk2001 is issued.

The description is:
Error LNK2001: unresolved external symbol "......."

I understand that this error occurs when a function lacking implementation or the compiler doesn't get the implementation for appropriate function. But the situation is different here. Because the source codes for the dll worked well when the writer wrote it. And furthermore, other member functions of the template class works fine without issuing any errors. In fact, these three functions do not be implemented indeed. But why was it correct, and not now?

Anybody who knows how to solove this kind of problem could tell me the how to?

Thank you very much!