I have a little muddle with the use of a class inside another class, and mixed with the dll perspective. (Althought I dont think this were the problem)

I have a DLL built successfully: MYCLASS_A.

I have a project, i want to use myclass.dll
I create an instance of it at my main.cpp.
Then, I use it using 'extern' .
All warks fine, I can use it in other cpp files.
But...
I want to have a MYCLASS_B (also into the dll) instantiated into MYCLASS_A, I want simply to write :
MYCLASS_A.MYCLASS_B.Myfunction.

I dont know how to write and use it...
The code :

myclass_a.h
Qt Code: 
  1. #ifndef MYCLASS_A_H
  2. #define MYCLASS_A_H
  3. #include "myclass_a_global.h"
  4. #include "myclass_b.h"
  5.  
  6. class MYCLASSASHARED_EXPORT Myclass_a
  7. {
  8. public:
  9.     Myclass_a();
  10.     Myclass_b myclass_b1;
  11. };
  12.  
  13. #endif
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myclass_a.cpp
Qt Code: 
  1. #include "myclass_a.h"
  2.  
  3. Myclass_a::Myclass_a()
  4. {
  5.  
  6. }
  7.  
  8. Myclass_b myclass_b1;
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When compile the project that use the DLL I have and 'undefined reference' to
myclass_a.myclass_b1.mymethod.
So, I suspect that the myclass_b1 is not really created ?
Please help....