Making a Class Readable from a Stream

Problem

You have written an object of some class to a stream, and now you need to read that data from the stream and use it to initialize an object of the same class.

Solution

Use operator>> to read data from the stream into your class to populate its data members, which is simply the reverse of what Example 10-6 does. See Example 10-7 for an implementation.

Example 10-7. Reading data into an object from a stream

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

using namespace std;

class Employee {
   friend ostream& operator<<              // These have to be friends
      (ostream& out, const Employee& emp); // so they can access
   friend istream& operator>>              // nonpublic members
      (istream& in, Employee& emp);

public:
   Employee( ) {}
  ~Employee( ) {}

   void setFirstName(const string& name) {firstName_ = name;}
   void setLastName(const string& name) {lastName_ = name;}

private:
   string firstName_;
   string lastName_;
};

// Send an Employee object to an ostream...
ostream& operator<<(ostream& out, const Employee& emp) {

   out << emp.firstName_ << endl;
   out << emp.lastName_ << endl;

   return(out);
}

// Read an Employee object from a stream
istream& operator>>(istream& in, Employee& emp) {

   in >> emp.firstName_;
   in >> emp.lastName_;

   return(in);
}

int main( ) {

   Employee emp;
   string first = "William";
   string last = "Shatner";

   emp.setFirstName(first);
   emp.setLastName(last);

   ofstream out("tmp\\emp.txt");

   if (!out) {
      cerr << "Unable to open output file.\n";
      exit(EXIT_FAILURE);
   }

   out << emp;  // Write the Emp to the file
   out.close( );

   ifstream in("tmp\\emp.txt");

   if (!in) {
      cerr << "Unable to open input file.\n";
      exit(EXIT_FAILURE);
   }

   Employee emp2;

   in >> emp2;  // Read the file into an empty object
   in.close( );

   cout << emp2;
}

Discussion

The steps for making a class readable from a stream are nearly identical to, but the opposite of, those for writing an object to a stream. If you have not already read Recipe 10.4, you should do so for Example 10-7 to make sense.

First, you have to declare an operator>> as a friend of your target class, but, in this case, you want it to use an istream instead of an ostream. Then define operator>> (instead of operator<<) to read values from the stream directly into each of your class's member variables. When you are done reading in data, return the input stream.