[Solved] Access predefined objects in header files

Add a forward declaration of library_class_1 in header.h before using it.

class library_class_1;

class header {
  public:
    header();
    void DoSomething(library_class_1 const&  object_1);
} 

Make sure to #include the .h file that defines library_class_1 in header.cc.

#include <librarry_class_1.h>

void header::DoSomething(library_class_1 & object_1)
{
    // Use object_1 ...
}