5.4.2 Alternative creation mechanisms

In cases where it is inappropriate to use the auto-detection mechanism an alternative way of initialising library with a pointer to a subclass of library_base must be provided. To do this, the implementation should provide subclasses of both library and library_base. The constructor of the subclass of library should call down to protected library::library (library_base* lb) constrctor with a newly allocated subclass of library_base. For example,

     class mylibrary : public library {
     private:
       class impl : public library_base {
         // override virtual functions
       };
     
       impl *init_library() {
         // Do any necessary initialisation
         return new impl;
       }
     
     public:
       mylibrary() : library( init_library() ) {}
     };

Usually it is sensible to avoid putting any data members or virtual functions in mylibrary, and to put them into mylibrary::impl instead. This means that if a mylibrary class is sliced to a library during a copy (for example when passing to a function taking a library by value) the class will still work correctly.

Note that library::~library is not virtual, and thus, if mylibrary is ever allocated on the heap, care is required to avoid deletion via a pointer to its base class.