How to pass and return object from C++ Functions?

In C++ programming, we can pass objects to a function in a similar manner as passing regular arguments.

Example 1: C++ Pass Objects to Function

// C++ program to calculate the average marks of two students

#include <iostream>
using namespace std;

class Student {

   public:
    double marks;

    // constructor to initialize marks
    Student(double m) {
        marks = m;
    }
};

// function that has objects as parameters
void calculateAverage(Student s1, Student s2) {

    // calculate the average of marks of s1 and s2 
    double average = (s1.marks + s2.marks) / 2;

   cout << "Average Marks = " << average << endl;

}

int main() {
    Student student1(88.0), student2(56.0);

  // pass the objects as arguments
   calculateAverage(student1, student2);

    return 0;
}

Run Code

Output

Average Marks = 72

Here, we have passed two Student objects student1 and student2 as arguments to the calculateAverage() function.

Pass objects to function in C++

---

Example 2: C++ Return Object from a Function

#include <iostream>
using namespace std;

class Student {
   public:
    double marks1, marks2;
};

// function that returns object of Student
Student createStudent() {
    Student student;

    // Initialize member variables of Student
    student.marks1 = 96.5;
    student.marks2 = 75.0;

    // print member variables of Student
    cout << "Marks 1 = " << student.marks1 << endl;
    cout << "Marks 2 = " << student.marks2 << endl;

    return student;
}

int main() {
    Student student1;

    // Call function
    student1 = createStudent();

    return 0;
}

Run Code

Output

Marks1 = 96.5
Marks2 = 75

Return object from function in C++

In this program, we have created a function createStudent() that returns an object of Student class.

We have called createStudent() from the main() method.

// Call function
student1 = createStudent();

Here, we are storing the object returned by the createStudent() method in the student1

 

How to pass and return object from C++ Functions?

In C++ programming, we can pass objects to a function in a similar manner as passing regular arguments.

Example 1: C++ Pass Objects to Function

// C++ program to calculate the average marks of two students

#include <iostream>
using namespace std;

class Student {

   public:
    double marks;

    // constructor to initialize marks
    Student(double m) {
        marks = m;
    }
};

// function that has objects as parameters
void calculateAverage(Student s1, Student s2) {

    // calculate the average of marks of s1 and s2 
    double average = (s1.marks + s2.marks) / 2;

   cout << "Average Marks = " << average << endl;

}

int main() {
    Student student1(88.0), student2(56.0);

  // pass the objects as arguments
   calculateAverage(student1, student2);

    return 0;
}

Run Code

Output

Average Marks = 72

Here, we have passed two Student objects student1 and student2 as arguments to the calculateAverage() function.

Pass objects to function in C++

---

Example 2: C++ Return Object from a Function

#include <iostream>
using namespace std;

class Student {
   public:
    double marks1, marks2;
};

// function that returns object of Student
Student createStudent() {
    Student student;

    // Initialize member variables of Student
    student.marks1 = 96.5;
    student.marks2 = 75.0;

    // print member variables of Student
    cout << "Marks 1 = " << student.marks1 << endl;
    cout << "Marks 2 = " << student.marks2 << endl;

    return student;
}

int main() {
    Student student1;

    // Call function
    student1 = createStudent();

    return 0;
}

Run Code

Output

Marks1 = 96.5
Marks2 = 75

Return object from function in C++

In this program, we have created a function createStudent() that returns an object of Student class.

We have called createStudent() from the main() method.

// Call function
student1 = createStudent();

Here, we are storing the object returned by the createStudent() method in the student1

 

C++ Constructors

A constructor is a special type of member function that is called automatically when an object is created.

In C++, a constructor has the same name as that of the class and it does not have a return type. For example,

class  Wall {
  public:
    // create a constructor
    Wall() {
      // code
    }
};

Here, the function Wall() is a constructor of the class Wall. Notice that the constructor

• has the same name as the class,
• does not have a return type, and
• is public

---

C++ Default Constructor

A constructor with no parameters is known as a default constructor. In the example above, Wall() is a default constructor.

---

Example 1: C++ Default Constructor

// C++ program to demonstrate the use of default constructor

#include <iostream>
using namespace std;

// declare a class
class  Wall {
  private:
    double length;

  public:
    // default constructor to initialize variable
    Wall() {
      length = 5.5;
      cout << "Creating a wall." << endl;
      cout << "Length = " << length << endl;
    }
};

int main() {
  Wall wall1;
  return 0;
}

Run Code

Output

Creating a Wall
Length = 5.5

Here, when the wall1 object is created, the Wall() constructor is called. This sets the length variable of the object to 5.5.

Note: If we have not defined a constructor in our class, then the C++ compiler will automatically create a default constructor with an empty code and no parameters.

---

C++ Parameterized Constructor

In C++, a constructor with parameters is known as a parameterized constructor. This is the preferred method to initialize member data.

---

Example 2: C++ Parameterized Constructor

// C++ program to calculate the area of a wall

#include <iostream>
using namespace std;

// declare a class
class Wall {
  private:
    double length;
    double height;

  public:
    // parameterized constructor to initialize variables
    Wall(double len, double hgt) {
      length = len;
      height = hgt;
    }

    double calculateArea() {
      return length * height;
    }
};

int main() {
  // create object and initialize data members
  Wall wall1(10.5, 8.6);
  Wall wall2(8.5, 6.3);

  cout << "Area of Wall 1: " << wall1.calculateArea() << endl;
  cout << "Area of Wall 2: " << wall2.calculateArea();

  return 0;
}

Run Code

Output

Area of Wall 1: 90.3
Area of Wall 2: 53.55

Here, we have created a parameterized constructor Wall() that has 2 parameters: double len and double hgt. The values contained in these parameters are used to initialize the member variables length and height.

When we create an object of the Wall class, we pass the values for the member variables as arguments. The code for this is:

Wall wall1(10.5, 8.6);
Wall wall2(8.5, 6.3);

With the member variables thus initialized, we can now calculate the area of the wall with the calculateArea() function.

---

C++ Copy Constructor

The copy constructor in C++ is used to copy data of one object to another.

---

Example 3: C++ Copy Constructor

#include <iostream>
using namespace std;

// declare a class
class Wall {
  private:
    double length;
    double height;

  public:

    // initialize variables with parameterized constructor
    Wall(double len, double hgt) {
      length = len;
      height = hgt;
    }

    // copy constructor with a Wall object as parameter
    // copies data of the obj parameter
    Wall(Wall &obj) {
      length = obj.length;
      height = obj.height;
    }

    double calculateArea() {
      return length * height;
    }
};

int main() {
  // create an object of Wall class
  Wall wall1(10.5, 8.6);

  // copy contents of wall1 to wall2
  Wall wall2 = wall1;

  // print areas of wall1 and wall2
  cout << "Area of Wall 1: " << wall1.calculateArea() << endl;
  cout << "Area of Wall 2: " << wall2.calculateArea();

  return 0;
}

Run Code

Output

Area of Wall 1: 90.3
Area of Wall 2: 90.3

In this program, we have used a copy constructor to copy the contents of one object of the Wall class to another. The code of the copy constructor is:

Wall(Wall &obj) {
  length = obj.length;
  height = obj.height;
}

Notice that the parameter of this constructor has the address of an object of the Wall class.

We then assign the values of the variables of the obj object to the corresponding variables of the object calling the copy constructor. This is how the contents of the object are copied.

In main(), we then create two objects wall1 and wall2 and then copy the contents of wall1 to wall2:

// copy contents of wall1 to wall2
Wall wall2 = wall1;

Here, the wall2 object calls its copy constructor by passing the address of the wall1 object as its argument i.e. &obj = &wall1.

Note: A constructor is primarily used to initialize objects. They are also used to run a default code when an object is created.

 

C++ Constructors

A constructor is a special type of member function that is called automatically when an object is created.

In C++, a constructor has the same name as that of the class and it does not have a return type. For example,

class  Wall {
  public:
    // create a constructor
    Wall() {
      // code
    }
};

Here, the function Wall() is a constructor of the class Wall. Notice that the constructor

• has the same name as the class,
• does not have a return type, and
• is public

---

C++ Default Constructor

A constructor with no parameters is known as a default constructor. In the example above, Wall() is a default constructor.

---

Example 1: C++ Default Constructor

// C++ program to demonstrate the use of default constructor

#include <iostream>
using namespace std;

// declare a class
class  Wall {
  private:
    double length;

  public:
    // default constructor to initialize variable
    Wall() {
      length = 5.5;
      cout << "Creating a wall." << endl;
      cout << "Length = " << length << endl;
    }
};

int main() {
  Wall wall1;
  return 0;
}

Run Code

Output

Creating a Wall
Length = 5.5

Here, when the wall1 object is created, the Wall() constructor is called. This sets the length variable of the object to 5.5.

Note: If we have not defined a constructor in our class, then the C++ compiler will automatically create a default constructor with an empty code and no parameters.

---

C++ Parameterized Constructor

In C++, a constructor with parameters is known as a parameterized constructor. This is the preferred method to initialize member data.

---

Example 2: C++ Parameterized Constructor

// C++ program to calculate the area of a wall

#include <iostream>
using namespace std;

// declare a class
class Wall {
  private:
    double length;
    double height;

  public:
    // parameterized constructor to initialize variables
    Wall(double len, double hgt) {
      length = len;
      height = hgt;
    }

    double calculateArea() {
      return length * height;
    }
};

int main() {
  // create object and initialize data members
  Wall wall1(10.5, 8.6);
  Wall wall2(8.5, 6.3);

  cout << "Area of Wall 1: " << wall1.calculateArea() << endl;
  cout << "Area of Wall 2: " << wall2.calculateArea();

  return 0;
}

Run Code

Output

Area of Wall 1: 90.3
Area of Wall 2: 53.55

Here, we have created a parameterized constructor Wall() that has 2 parameters: double len and double hgt. The values contained in these parameters are used to initialize the member variables length and height.

When we create an object of the Wall class, we pass the values for the member variables as arguments. The code for this is:

Wall wall1(10.5, 8.6);
Wall wall2(8.5, 6.3);

With the member variables thus initialized, we can now calculate the area of the wall with the calculateArea() function.

---

C++ Copy Constructor

The copy constructor in C++ is used to copy data of one object to another.

---

Example 3: C++ Copy Constructor

#include <iostream>
using namespace std;

// declare a class
class Wall {
  private:
    double length;
    double height;

  public:

    // initialize variables with parameterized constructor
    Wall(double len, double hgt) {
      length = len;
      height = hgt;
    }

    // copy constructor with a Wall object as parameter
    // copies data of the obj parameter
    Wall(Wall &obj) {
      length = obj.length;
      height = obj.height;
    }

    double calculateArea() {
      return length * height;
    }
};

int main() {
  // create an object of Wall class
  Wall wall1(10.5, 8.6);

  // copy contents of wall1 to wall2
  Wall wall2 = wall1;

  // print areas of wall1 and wall2
  cout << "Area of Wall 1: " << wall1.calculateArea() << endl;
  cout << "Area of Wall 2: " << wall2.calculateArea();

  return 0;
}

Run Code

Output

Area of Wall 1: 90.3
Area of Wall 2: 90.3

In this program, we have used a copy constructor to copy the contents of one object of the Wall class to another. The code of the copy constructor is:

Wall(Wall &obj) {
  length = obj.length;
  height = obj.height;
}

Notice that the parameter of this constructor has the address of an object of the Wall class.

We then assign the values of the variables of the obj object to the corresponding variables of the object calling the copy constructor. This is how the contents of the object are copied.

In main(), we then create two objects wall1 and wall2 and then copy the contents of wall1 to wall2:

// copy contents of wall1 to wall2
Wall wall2 = wall1;

Here, the wall2 object calls its copy constructor by passing the address of the wall1 object as its argument i.e. &obj = &wall1.

Note: A constructor is primarily used to initialize objects. They are also used to run a default code when an object is created.

 

C++ Classes and Objects

In previous tutorials, we learned about functions and variables. Sometimes it's desirable to put related functions and data in one place so that it's logical and easier to work with.

Suppose, we need to store the length, breadth, and height of a rectangular room and calculate its area and volume.

To handle this task, we can create three variables, say, length, breadth, and height along with the functions calculateArea() and calculateVolume().

However, in C++, rather than creating separate variables and functions, we can also wrap these related data and functions in a single place (by creating objects). This programming paradigm is known as object-oriented programming.

But before we can create objects and use them in C++, we first need to learn about classes.

---

C++ Class

A class is a blueprint for the object.

We can think of a class as a sketch (prototype) of a house. It contains all the details about the floors, doors, windows, etc. Based on these descriptions we build the house. House is the object.

Create a Class

A class is defined in C++ using keyword class followed by the name of the class.

The body of the class is defined inside the curly brackets and terminated by a semicolon at the end.

class className {
   // some data
   // some functions
};

For example,

class Room {
    public:
        double length;
        double breadth;
        double height;   

        double calculateArea(){   
            return length * breadth;
        }

        double calculateVolume(){   
            return length * breadth * height;
        }

};

Here, we defined a class named Room.

The variables length, breadth, and height declared inside the class are known as data members. And, the functions calculateArea() and calculateVolume() are known as member functions of a class.

---

C++ Objects

When a class is defined, only the specification for the object is defined; no memory or storage is allocated.

To use the data and access functions defined in the class, we need to create objects.

---

Syntax to Define Object in C++

className objectVariableName;

We can create objects of Room class (defined in the above example) as follows:

// sample function
void sampleFunction() {
    // create objects
    Room room1, room2;
}

int main(){
    // create objects 
    Room room3, room4;
}

Here, two objects room1 and room2 of the Room class are created in sampleFunction(). Similarly, the objects room3 and room4 are created in main().

As we can see, we can create objects of a class in any function of the program. We can also create objects of a class within the class itself, or in other classes.

Also, we can create as many objects as we want from a single class.

---

C++ Access Data Members and Member Functions

We can access the data members and member functions of a class by using a . (dot) operator. For example,

room2.calculateArea();

This will call the calculateArea() function inside the Room class for object room2.

Similarly, the data members can be accessed as:

room1.length = 5.5;

In this case, it initializes the length variable of room1 to 5.5.

---

Example 1: Object and Class in C++ Programming

// Program to illustrate the working of
// objects and class in C++ Programming

#include <iostream>
using namespace std;

// create a class
class Room {

   public:
    double length;
    double breadth;
    double height;

    double calculateArea() {
        return length * breadth;
    }

    double calculateVolume() {
        return length * breadth * height;
    }
};

int main() {

    // create object of Room class
    Room room1;

    // assign values to data members
    room1.length = 42.5;
    room1.breadth = 30.8;
    room1.height = 19.2;

    // calculate and display the area and volume of the room
    cout << "Area of Room =  " << room1.calculateArea() << endl;
    cout << "Volume of Room =  " << room1.calculateVolume() << endl;

    return 0;
}

Run Code

Output

Area of Room =  1309
Volume of Room =  25132.8

In this program, we have used the Room class and its object room1 to calculate the area and volume of a room.

In main(), we assigned the values of length, breadth, and height with the code:

room1.length = 42.5;
room1.breadth = 30.8;
room1.height = 19.2;

We then called the functions calculateArea() and calculateVolume() to perform the necessary calculations.

Note the use of the keyword public in the program. This means the members are public and can be accessed anywhere from the program.

As per our needs, we can also create private members using the private keyword. The private members of a class can only be accessed from within the class. For example,
 

class Test {

private:
    int a;
    void function1() { }

public:
    int b;
    void function2() { }
}

Here, a and function1() are private. Thus they cannot be accessed from outside the class.

On the other hand, b and function2() are accessible from everywhere in the program.

To learn more about public and private keywords, please visit our C++ Class Access Modifiers tutorial.

---

Example 2: Using public and private in C++ Class

// Program to illustrate the working of
// public and private in C++ Class

#include <iostream>
using namespace std;

class Room {

   private:
    double length;
    double breadth;
    double height;

   public:

    // function to initialize private variables
    void initData(double len, double brth, double hgt) {
        length = len;
        breadth = brth;
        height = hgt;
    }

    double calculateArea() {
        return length * breadth;
    }

    double calculateVolume() {
        return length * breadth * height;
    }
};

int main() {

    // create object of Room class
    Room room1;

    // pass the values of private variables as arguments
    room1.initData(42.5, 30.8, 19.2);

    cout << "Area of Room =  " << room1.calculateArea() << endl;
    cout << "Volume of Room =  " << room1.calculateVolume() << endl;

    return 0;
}

Run Code

Output

Area of Room =  1309
Volume of Room =  25132.8

The above example is nearly identical to the first example, except that the class variables are now private.

Since the variables are now private, we cannot access them directly from main(). Hence, using the following code would be invalid:

// invalid code
obj.length = 42.5;
obj.breadth = 30.8;
obj.height = 19.2;

Instead, we use the public function initData() to initialize the private variables via the function parameters double len, double brth, and double hgt.

 

C++ Classes and Objects

In previous tutorials, we learned about functions and variables. Sometimes it's desirable to put related functions and data in one place so that it's logical and easier to work with.

Suppose, we need to store the length, breadth, and height of a rectangular room and calculate its area and volume.

To handle this task, we can create three variables, say, length, breadth, and height along with the functions calculateArea() and calculateVolume().

However, in C++, rather than creating separate variables and functions, we can also wrap these related data and functions in a single place (by creating objects). This programming paradigm is known as object-oriented programming.

But before we can create objects and use them in C++, we first need to learn about classes.

---

C++ Class

A class is a blueprint for the object.

We can think of a class as a sketch (prototype) of a house. It contains all the details about the floors, doors, windows, etc. Based on these descriptions we build the house. House is the object.

Create a Class

A class is defined in C++ using keyword class followed by the name of the class.

The body of the class is defined inside the curly brackets and terminated by a semicolon at the end.

class className {
   // some data
   // some functions
};

For example,

class Room {
    public:
        double length;
        double breadth;
        double height;   

        double calculateArea(){   
            return length * breadth;
        }

        double calculateVolume(){   
            return length * breadth * height;
        }

};

Here, we defined a class named Room.

The variables length, breadth, and height declared inside the class are known as data members. And, the functions calculateArea() and calculateVolume() are known as member functions of a class.

---

C++ Objects

When a class is defined, only the specification for the object is defined; no memory or storage is allocated.

To use the data and access functions defined in the class, we need to create objects.

---

Syntax to Define Object in C++

className objectVariableName;

We can create objects of Room class (defined in the above example) as follows:

// sample function
void sampleFunction() {
    // create objects
    Room room1, room2;
}

int main(){
    // create objects 
    Room room3, room4;
}

Here, two objects room1 and room2 of the Room class are created in sampleFunction(). Similarly, the objects room3 and room4 are created in main().

As we can see, we can create objects of a class in any function of the program. We can also create objects of a class within the class itself, or in other classes.

Also, we can create as many objects as we want from a single class.

---

C++ Access Data Members and Member Functions

We can access the data members and member functions of a class by using a . (dot) operator. For example,

room2.calculateArea();

This will call the calculateArea() function inside the Room class for object room2.

Similarly, the data members can be accessed as:

room1.length = 5.5;

In this case, it initializes the length variable of room1 to 5.5.

---

Example 1: Object and Class in C++ Programming

// Program to illustrate the working of
// objects and class in C++ Programming

#include <iostream>
using namespace std;

// create a class
class Room {

   public:
    double length;
    double breadth;
    double height;

    double calculateArea() {
        return length * breadth;
    }

    double calculateVolume() {
        return length * breadth * height;
    }
};

int main() {

    // create object of Room class
    Room room1;

    // assign values to data members
    room1.length = 42.5;
    room1.breadth = 30.8;
    room1.height = 19.2;

    // calculate and display the area and volume of the room
    cout << "Area of Room =  " << room1.calculateArea() << endl;
    cout << "Volume of Room =  " << room1.calculateVolume() << endl;

    return 0;
}

Run Code

Output

Area of Room =  1309
Volume of Room =  25132.8

In this program, we have used the Room class and its object room1 to calculate the area and volume of a room.

In main(), we assigned the values of length, breadth, and height with the code:

room1.length = 42.5;
room1.breadth = 30.8;
room1.height = 19.2;

We then called the functions calculateArea() and calculateVolume() to perform the necessary calculations.

Note the use of the keyword public in the program. This means the members are public and can be accessed anywhere from the program.

As per our needs, we can also create private members using the private keyword. The private members of a class can only be accessed from within the class. For example,
 

class Test {

private:
    int a;
    void function1() { }

public:
    int b;
    void function2() { }
}

Here, a and function1() are private. Thus they cannot be accessed from outside the class.

On the other hand, b and function2() are accessible from everywhere in the program.

To learn more about public and private keywords, please visit our C++ Class Access Modifiers tutorial.

---

Example 2: Using public and private in C++ Class

// Program to illustrate the working of
// public and private in C++ Class

#include <iostream>
using namespace std;

class Room {

   private:
    double length;
    double breadth;
    double height;

   public:

    // function to initialize private variables
    void initData(double len, double brth, double hgt) {
        length = len;
        breadth = brth;
        height = hgt;
    }

    double calculateArea() {
        return length * breadth;
    }

    double calculateVolume() {
        return length * breadth * height;
    }
};

int main() {

    // create object of Room class
    Room room1;

    // pass the values of private variables as arguments
    room1.initData(42.5, 30.8, 19.2);

    cout << "Area of Room =  " << room1.calculateArea() << endl;
    cout << "Volume of Room =  " << room1.calculateVolume() << endl;

    return 0;
}

Run Code

Output

Area of Room =  1309
Volume of Room =  25132.8

The above example is nearly identical to the first example, except that the class variables are now private.

Since the variables are now private, we cannot access them directly from main(). Hence, using the following code would be invalid:

// invalid code
obj.length = 42.5;
obj.breadth = 30.8;
obj.height = 19.2;

Instead, we use the public function initData() to initialize the private variables via the function parameters double len, double brth, and double hgt.