Classes and Objects

A class is the foundation of C++ design. It keeps data private and exposes only the operations that maintain its rules.

A Class

Members are private by default in a class. Expose public methods that preserve the object's rules.

#include <iostream>
#include <string>

class Student {
private:
    std::string name;
    int score;

public:
    Student(std::string name, int score)
        : name(std::move(name)), score(score) {}

    void addScore(int points) {
        if (points > 0) score += points;
    }

    std::string grade() const {
        if (score >= 70) return "A";
        if (score >= 60) return "B";
        if (score >= 50) return "C";
        return "F";
    }

    const std::string& getName() const {
        return name;
    }
};

Constructors

A constructor sets up a valid object. Use a member initialiser list to initialise fields directly instead of assigning inside the body.

Student student("Amina", 64);
student.addScore(8);

std::cout << student.getName() << ": " << student.grade() << "\n";

const Correctness

Mark methods that do not change the object as const. It lets callers use the object through a const reference and documents intent.

Practice

1. Create a Book class with title, author and price.
2. Add a method that returns a formatted description.
3. Reject a negative price in the constructor.

Hands-on exercise

Create a BankAccount class with:
1. A private balance.
2. A constructor that accepts an opening balance.
3. deposit and withdraw methods.
4. withdraw refuses to go below zero.
5. A getBalance method marked const.

Recap

Classes keep data private and expose safe operations. Constructors create valid objects and const methods document read-only behaviour.