Expert: vijayan - 8/22/2008

Question
QUESTION: Hi:

I want to create a class object by reading the text file. For example, if my text file has sentence "className A", then it should create that object A on the fly, again I also able to create multiple objects of same class by reading text file as explained above.

Can I do that?

Thanks,
Best Regards

ANSWER: in a simple scenario, you could do something like this:

#include <iostream>
#include <fstream>
#include <string>

class base
{
 public:
     virtual ~base() {}
     virtual void save_state( std::ostream& stm ) const = 0 ;
     virtual void  restore_state( std::ostream& stm ) = 0 ;
     virtual const std::string&  identifier() const = 0 ;
};

//---------------------------------------------------------------------------------------

class A : public base
{
  public:
     A( std::istream& ) ;
     virtual void save_state( std::ostream& stm ) const
     {
       // write the state of the object to stm
     }
       virtual void  restore_state( std::ostream& stm )
     {
       // read the state from stm , set of the object to read values
     }
       virtual const std::string&  identifier() const { return id ; }

       static const std::string id ;

       // other stuff
};

const std::string A::id = "className A" ; // unique id for class A

//---------------------------------------------------------------------------------------

class B : public base
{
  public:
     B( std::istream& ) ;
     virtual void save_state( std::ostream& stm ) const
     {
       // write the state of the object to stm
     }
       virtual void  restore_state( std::ostream& stm )
     {
       // read the state from stm , set of the object to read values
     }
       virtual const std::string&  identifier() const { return id ; }

      static const std::string id ;

       // other stuff
};

const std::string B::id = "className B" ; // unique id for class B

//---------------------------------------------------------------------------------------

void save( const base* object, std::ostream& stm )
{
 stm << object->identifier() << '\n' ;
 object->save_state(stm) ;
}

base* create_from_stream( std::istream& stm )
{
 base* object = 0 ;
 std::string id ;
 std::getline( stm, id ) ;
 if( id == A::id )   return new A(stm) ;
 else if( id == B::id )   return new B(stm) ;
 // etc
 return 0 ;
}

if you need more abstraction than this, use an exemplar (prototype) design pattern.
a very good discussion on implementing exemplars in C++ is available in:
Advanced C++ Programming Styles and Idioms by James O. Coplien
http://www.amazon.com/Advanced-C-Programming-Styles-Idioms/dp/0201548550
see chapter 8: Programming with Exemplars in C++ pages 279-306


---------- FOLLOW-UP ----------

QUESTION: Thanks for your quick reply. I am really appreciated.

I have couple of doubts:
1) where could call of the void save(..) and base*   create_from_stream(..) lie? I mean which is the calling function, is it main()?

2)What are save_state and restore_state functions meant for?

Thanks,
Best Regards,
Dipen Patel


Answer
> where could call of the void save(..) and base*   create_from_stream(..) lie?
anywhere, really. if you are saving the objects into a file when your program exits and recreating the objects on restart, main would be a logical choice.

> What are save_state and restore_state functions meant for?
save_state should write out information about the object in such a way that the information can be read back later.
restore_state is the function that actually reads the saved information and modifies the member variables so that they corrospond to the state saved earlier.

for example:

class base
{
 public:
     virtual ~base() {}
     virtual void save_state( std::ostream& stm ) const = 0 ;
     virtual void  restore_state( std::istream& stm ) = 0 ;
     virtual const std::string&  identifier() const = 0 ;
};


class Book : public base
{
  public:
     // constructor
     Book( const std::string& _title, const std::string& _author, int _npages )
         : title(_title), author(_author), npages(_npages) {}

     // constructor to initialize from stream
     Book( std::istream& stm ) {  restore_state(stm) ; }

     // write the state of the object to stm
     virtual void save_state( std::ostream& stm ) const
     {
       // write out each data member to stream
       // seperated by a newline
       stm << title << '\n' << author << '\n' << npages << '\n' ;
     }

     // read the state from stm , set of the object to read values
     virtual void  restore_state( std::istream& stm )
     {
       // read each data member from stream (seperated by a newline)
       // and set the values to member variables
       std::string str ;
       std::getline( stm, str ) ; title = str ;
       std::getline( stm, str ) ; author = str ;
       int n ;
       stm >> n >> std::ws ;
       npages = n ;
     }

     virtual const std::string&  identifier() const { return id ; }

      static const std::string id ;
      
      // other members
 private:
   std::string title ;
   std::string author ;
   int npages ;
};

const std::string Book::id = "className Book" ; // unique id for class Book

notes:
1. error handling has been elided for brevity.
2. save_state should take an input stream
  and restore_state should take an input stream (std::istream)
3. you could construct a std::ofstream to save to a file and a std::ifstream to read back from that file.