In this article we will discuss how to write objects to file and how to read them back from to file and fill in object.

Suppose we have a class Student i.e.

class Student
{
public:
	std::string mName;
	std::string mId;
	std::string mPhoneNumber;

	Student(std::string id = "", std::string name = "", std::string phone = "") : mId(id), mName(name), mPhoneNumber(phone)
	{}

	bool operator==(const Student & obj)
	{
		return (mId == obj.mId) && (mName == obj.mName) && (mPhoneNumber == obj.mPhoneNumber);
	}

};

Writing objects in File | Serializing Objects

To write an object in file we need to over load << operator for the file as friend function i.e.

/*
 * Write the member variables to stream objects
 */
friend std::ostream & operator << (std::ostream &out, const Student & obj)
{
	out << obj.mId << "\n" <<obj.mName<<"\n"<<obj.mPhoneNumber<<std::endl;
	return out;
}

Now we can create Student class objects and store them in file i.e.
Student stud1("1","Jack", "4445554455");
Student stud2("4","Riti", "4445511111");
Student stud3("6","Aadi", "4040404011");

// Open the File
std::ofstream out("students.txt");

// Write objects to file
out<<stud1;
out<<stud2;
out<<stud3;

out.close();

Contents of file students.txt will be,
1
Jack
4445554455
4
Riti
4445511111
6
Aadi
4040404011

Reading objects from File | Deserializing Objects

To read an object in file we need to overload >> operator for the file as friend function i.e.

/*
 * Read data from stream object and fill it in member variables
 */
friend std::istream & operator >> (std::istream &in,  Student &obj)
{
	in >> obj.mId;
	in >> obj.mName;
	in >> obj.mPhoneNumber;
	return in;
}

Now we can load data in Student class objects from file i.e.
// Open the File
std::ifstream in("students.txt");

Student student1;
Student student2;
Student student3;

// Read objects from file and fill in data
in>>student1;
in>>student2;
in>>student3;

in.close();

Checkout complete example as follows,
#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <cassert>

class Student
{
public:
	std::string mName;
	std::string mId;
	std::string mPhoneNumber;

	Student(std::string id = "", std::string name = "", std::string phone = "") : mId(id), mName(name), mPhoneNumber(phone)
	{}

	bool operator==(const Student & obj)
	{
		return (mId == obj.mId) && (mName == obj.mName) && (mPhoneNumber == obj.mPhoneNumber);
	}

	/*
	 * Write the member variables to stream objects
	 */
	friend std::ostream & operator << (std::ostream &out, const Student & obj)
	{
		out << obj.mId << "\n" <<obj.mName<<"\n"<<obj.mPhoneNumber<<std::endl;
		return out;
	}
	/*
	 * Read data from stream object and fill it in member variables
	 */
	friend std::istream & operator >> (std::istream &in,  Student &obj)
	{
		in >> obj.mId;
		in >> obj.mName;
		in >> obj.mPhoneNumber;
		return in;
	}


};

int main()
{
	Student stud1("1","Jack", "4445554455");
	Student stud2("4","Riti", "4445511111");
	Student stud3("6","Aadi", "4040404011");

	// Open the File
	std::ofstream out("students.txt");

	// Write objects to file
	out<<stud1;
	out<<stud2;
	out<<stud3;

	out.close();

	// Open the File
	std::ifstream in("students.txt");

	Student student1;
	Student student2;
	Student student3;

	// Read objects from file and fill in data
	in>>student1;
	in>>student2;
	in>>student3;

	in.close();

	// Compare the Objects
	assert(stud1==student1);
	assert(stud2==student2);
	assert(stud3==student3);

	return 0;
}

To compile the above example in linux use following command,

g++ –std=c++11 example.cpp

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