Concept of class with example

A class is a user-defined data type that groups data members (attributes) and member functions (behaviour) into a single unit. It is a blueprint; no memory is allocated for it until objects are created. Members can be private, protected or public; by default members of a class are private.

General form

class ClassName {
private:
    // data members (hidden)
public:
    // member functions (interface)
};   // semicolon is required

Example

#include <iostream>
using namespace std;

class Student {
private:
    int roll;
    float marks;
public:
    void getData(int r, float m);      // declaration
    void display() {                   // defined inside the class (inline)
        cout << "Roll: " << roll << ", Marks: " << marks << endl;
    }
};

void Student::getData(int r, float m) { // defined outside using ::
    roll = r;
    marks = m;
}

int main() {
    Student s1, s2;                     // objects
    s1.getData(1, 78.5);
    s2.getData(2, 85);
    s1.display();
    s2.display();
    return 0;
}

Output

Roll: 1, Marks: 78.5
Roll: 2, Marks: 85

Points:

• Each object has its own copy of the data members; member functions are shared by all objects.
• Private members are reached only through public functions (data hiding).
• A member function can be defined inside the class, or outside using the scope resolution operator ::.
• Objects are accessed with the dot operator (s1.display()).