c++ - separating constructor implementation with template from header file

My header file has

template 
class AA : public BB
{

public:
    AA()
    { ... }

this is working fine. But I need to separate the constructor implementation from header file.

So in cpp, I have

template 

AA::AA()
{ ... }

When I compile this, it compiled but I get unresolved external symbol error. What am I missing here?

Answer

You can explicitly instantiate templates in your implementation file using :

template class AA;

this will generate a definition from a template, but it is of use only if you know what types your class clients will use