Manipulating Streams and Files with C++

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.

See Also

Recipe 10.4

Please check back next week for the continuation of this article.