Classes vs Structure vs Union in C++

Class: It is a user-defined datatype enclosed with variables and functions. It is like a blueprint for an object. Class members are private by default. For Example, the car is an object, its color, design, weight are its attributes whereas the brake, speed limit, etc. are its functions.

Syntax:

class <class_name>
{
     private:
     // Data members
     ........... ...... .......
     
     public:
     // Data members
     // Member function
     ......... ........ ......
   
     protected:
     // Data members
     // Member function
    ....... ....... ......
};

Below is the C++ program to illustrate the class:

C++

// C++ program to illustrate class
#include <bits/stdc++.h>
using namespace std;

// Class with data members
// and member function
class Geeks {
public:
    string geekname;
    int roll_no;

    // Function printName
    // prints the name
    void printName()
    {
        cout << "Geekname is: "
             << geekname;
    }

    // Function printRollno
    // prints the roll no
    void printRollno()
    {
        cout << "Roll no  is: "
             << roll_no;
    }
};

// Driver Code
int main()
{

    // Object of class Geeks
    Geeks obj1;

    // Initialize a public data
    // members of class Geeks
    obj1.geekname = "Geek";
    obj1.roll_no = 15;

    // Function Call
    obj1.printName();
    cout << endl;

    // Function Call
    obj1.printRollno();

    return 0;
}

Output:

Geekname is: Geek
Roll no  is: 15

Explanation: In the above example of C++ class Geeks has data members and member function in public access specifier. The data members are initialized with some values using the object of the class. Using the object, the functions of the class are being called.

Structure: It is a convenient way for grouping variables. Structure members are public members by default. Structures can be used to effectively raise the level of abstraction in order to improve the code.

Syntax:

struct  <struct_name>
{
    // Data members
    ....... ...... ......
       
    // Member functions
    ....... ...... ......
};

Below is the C++ program to illustrate structure:

C++

// C++ program to illustrate structure
#include <bits/stdc++.h>
using namespace std;

// Structure with data members
// and member function
struct geekyPerson {

    // Data members
    string name;
    int age;
    float salary;

    // Member function to display
    void display()
    {
        cout << "Name: " << name
             << endl;
        cout << "Age: " << age
             << endl;
        cout << "Salary: " << salary;
    }
};

// Driver Code
int main()
{
    // Instance of the structure
    geekyPerson g1;

    // Accessing the data by the
    // dot operator
    g1.name = "Geeky";
    g1.age = 23;
    g1.salary = 15000;

    // Function Call
    g1.display();

    return 0;
}

Output:

Name: Geeky
Age: 23
Salary: 15000

Explanation: The struct tag is used before the structure name to define a structure. Each member definition is a normal variable definition, such as int i; or float f; or any other valid variable definition. Using the object, the functions of the structure are being called.

Union: It is a user-defined datatype. It is similar to structures. All the data members of the union share the same memory location. The size of the union is based on the size of the largest member of the union. To use the same memory location for two or more members, a union will be the best.

Syntax:

union <union_name>
{
    // Data members
    ..... ....... .........
} union_variables;
 
where,
union_name − Name was given to the union.
union_variable − The object of union.
Data members -Member variables.

Below is the C++ program to illustrate union:

C++

// C++ program to illustrate union
#include <bits/stdc++.h>
using namespace std;

// Union declaration with the
// data members
union geeks {
    int id;
    int salary;
    char name[30];
    int age;
};

// Driver Code
int main()
{
    // Instance of the union
    union geeks g1;

    g1.id = 1;
    cout << "Id  : " << g1.id
         << endl;

    strcpy(g1.name, "Geeky");
    cout << "Name : " << g1.name
         << endl;

    g1.salary = 35000;
    cout << "Salary : " << g1.salary
         << endl;

    g1.age = 25;
    cout << "Age : " << g1.age
         << endl;

    return 0;
}

Output:

Id  : 1
Name : Geeky
Salary : 35000
Age : 25

Explanation: In union also, objects are used to access the data members and member functions.