Making a Class Readable from a Stream

2017-11-03 09:05:03

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 #include #include #include 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. "; 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. "; 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.

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