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Creating Truly Maintainable Class Factories

November 2000/Creating Truly Maintainable Class Factories/Listing 2

Listing 2: A naive implementation of a DLL-based class factory

#include <string>
#include <exception>
#include <assert.h>

#include <windows.h>

#include "listing1.h"

class foo_factory
      bool invariant() 
         { return m_h_dll != 0 }

         ( std::string const& dll_name 
         , std::string const& prefix ) throw( exception )
         : m_h_dll( NULL ), m_prefix( prefix )
         HINSTANCE h_dll = LoadLibrary( dll_name.c_str() );
         if ( !h_dll )
            throw exception;
         m_h_dll = h_dll;
         assert( invariant() );

      ~foo_factory( )
         assert( invariant() );
         FreeLibrary( m_h_dll );

      foo* instantiate( std::string const& req_class_id )
         assert( invariant() );
         typedef foo* (DLLCALL* pfn_factory_entry)( );

         std::string entry_name( decorate( req_class_id ) );
         pfn_factory_entry pfn_instantiate = 
            GetProcAddress( m_h_dll , entry_name.c_str() );
         if ( !pfn_instantiate )
            throw exception;

         return (pfn_instantiate)( );

      std::string decorate( std::string const& req_class_id )
         // '_' is necessary because of DLLCALL's name-mangling
         // effects on Win32 compilers.
         return "_" + m_prefix + req_class_id;

      HINSTANCE    m_h_dll;
      std::string m_prefix;

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