Expert: vijayan - 2/2/2009

Question
i have writen a class and then i have writen another class using an object of the first class! now i want to save an object of the second class on a file!

class table{
 public:
  vector< vector<string> >


and then the other one:
class project{
  public:
     vector<table> db;
now the object of class project that i wantt to save is named dataBase.
how should i do that?
i need a piece of code!
pleaze answer az soon az possible! i need it awfully early!
thank you!

Answer
use a std::ofstream to write to a file and a std::ifstream to read from a file. see: http://www.cplusplus.com/doc/tutorial/files.html

since the primary underlying data that you need to read or write is a string, the simplest way is to use a text file and write them out one string per line of text. to read a complete line from an input stream into a C++ std::string, use std::getline. http://www.cplusplus.com/reference/string/getline.html

writing a vector of strings to an output stream is easy:
a. write the number of strings
b. write the strings, one per line.

#include <vector>
#include <string>
#include <iostream>
#include <fstream>
using namespace std ;

void save_vector_of_strings( vector<string>& vec,  ostream& stream )
{
   stream << vec.size() << '\n' ;  // write out number of strings in the vector

   // write the strings one string per line
   for( size_t i = 0 ; i < vec.size() ; ++i )
       stream << vec[i] << '\n' ;
}

and to read it back:

bool load_vector_of_strings( vector<string>& vec,  istream& stream )
{
   vec.clear() ; // make it an empty vector

   std::size_t n = 0 ;
   stream >> n >> ws ; // read number of strings in the vector, discard terminating newline
   if( !stream ) return false ; // read failed

     // read the strings one string per line and add them to the vector
     string line ;
     for( size_t i = 0 ; i < n ; ++i )
         if( getline( stream, line ) )  vec.push_back( line ) ;

     return true ; // success
}

a table is just a vector of vectors, so:

class table
{
   public:
       vector< vector<string> > data ;
       
       void save_table( ostream& stream )
       {
           stream << data.size() << '\n' ;  // write out number of vectors

           // write the vectors
           for( size_t i = 0 ; i < data.size() ; ++i )
               save_vector_of_strings( data[i], stream ) ;    
       }

       bool load_table( istream& stream )
       {
           std::size_t n = 0 ;
           stream >> n >> ws ; // read number of vectors
           if( !stream ) return false ; // read failed

             // read the vectors one by one and add them to the vector
             vector<string> strings ;
             for( size_t i = 0 ; i < n ; ++i )
             {
                 if( load_vector_of_strings( strings, stream ) )  data.push_back( strings ) ;
             }

             return true ; // success
       }

       // ...
};

a project is a vector of tables, so the same approch would extend to that too.