5.4.3 Subclassing library_base

In addition to providing mechanism for opening libraries, implementations of new library types must also override at least the library_base::good and library_base::begin virtual functions. The implementation of the good function is usually trivial, and begin only slightly less so. Continuing with the example used in Supporting auto-detection, they might be as follows:

     class mylibrary : public library_base {
       mylibrary(const string &filename) : ifs(filename) {}
     
       class entry_type : public library_entry {
         // ...
       };
     
       const_iterator make_begin() {
         ifs.clear(); ifs.seekg(0, ios::beg); // Rewind the stream
         return const_iterator( this, new entry_type );
       }
     
       virtual bool good() const { return ifs; }
     
       virtual const_iterator begin() const {
         return const_cast< mylibrary* >( this )->make_begin();
       }
     
       ifstream ifs;
     }

Notice the slight awkwardness of the const_cast in begin. This is necessary because the begin function is sematically constant—it does not change the visible state of the library—but it needs to rewind ifs. The stream's position is not part of the visible state of the library because the library::const_iterator is only an Input Iterator and so only one of them can ever be present at any time.