IntermediatePractice in the editor

C++ Classes and Objects

A class is a plan for a kind of object: it lists the data each object holds and the functions that work on it. An object is one thing made from that plan. std::string is a class; every string variable is an object of it.

Defining a class

ExampleTry it in the editor

#include <iostream>
#include <string>

class Car {
public:
    std::string brand;
    int year = 0;
    int speed = 0;

    void accelerate(int amount) {
        speed += amount;
    }
    void describe() const {
        std::cout << brand << " (" << year << ") at " << speed << " km/h\n";
    }
};

int main() {
    Car a;
    a.brand = "Tata";
    a.year = 2022;
    a.accelerate(40);

    Car b;
    b.brand = "Maruti";
    b.year = 2019;
    b.accelerate(25);
    b.accelerate(30);

    a.describe();
    b.describe();
    return 0;
}

Output

Tata (2022) at 40 km/h
Maruti (2019) at 55 km/h

Data members (brand, year, speed) are the object's data; every object has its own copy.
Member functions (accelerate, describe) work on the object they are called on, with a dot: a.accelerate(40).
const after describe() promises that it only reads the object.
A class definition ends with };, with a semicolon.

public and private

Members under public: can be used from anywhere; members under private: only by the class's own functions. Class members are private by default. Good classes keep their data private and offer public functions:

ExampleTry it in the editor

#include <iostream>
#include <string>

class Student {
private:
    std::string name;
    int marks = 0;
public:
    void setName(const std::string& n) { name = n; }
    void setMarks(int m) {
        if (m >= 0 && m <= 100) marks = m;   // the class checks the value
    }
    std::string getName() const { return name; }
    int getMarks() const { return marks; }
};

int main() {
    Student s;
    s.setName("Asha");
    s.setMarks(84);
    s.setMarks(150);                 // refused: outside 0 to 100
    std::cout << s.getName() << ": " << s.getMarks() << "\n";
    return 0;
}

Output

Asha: 84

Functions like setMarks and getMarks are called setters and getters. The setter can refuse wrong values, so a Student can never hold impossible marks.

Objects in a vector

ExampleTry it in the editor

#include <iostream>
#include <string>
#include <vector>

class Item {
public:
    std::string name;
    double price = 0;
    int qty = 0;
    double total() const { return price * qty; }
};

int main() {
    std::vector<Item> cart = {{"Pen", 10, 3}, {"Notebook", 45, 2}};
    double sum = 0;
    for (const Item& it : cart) {
        std::cout << it.name << ": " << it.total() << "\n";
        sum += it.total();
    }
    std::cout << "Cart total: " << sum << "\n";
    return 0;
}

Output

Pen: 30
Notebook: 90
Cart total: 120

Class or struct?

class and struct differ only in the default: members of a class are private unless you write public:, members of a struct are public. By convention, struct is used for plain groups of data and class for types that protect their data and have rules, like Student above.

Practice exercises

Write a Rectangle class with width and height and member functions area and perimeter.
Make the members private and add setters that refuse negative sizes.
Store three rectangles in a vector and print the one with the largest area.