My issue was: I had to do forward declaration of the class whose ctor was "unresolved external".
In the file where I got the error, I had to put something like this:
#include "ClassB"
class ClassB; // this solved the problem
class ClassA{
void foo(){
ClassB* tmp = new ClassB();
// ...
}
};
Of course, my project is much more complicated and this is just a snippet example. Also when using namespaces, declare them as well.