Encapsulation

Marking a member private prevents code outside the class from touching it directly — the class can then guarantee its own invariants, since the only way to change its state is through the methods it chooses to expose. A method marked const promises not to modify the object it's called on, which the compiler checks and enforces.

#include <iostream>
using namespace std;

class BankAccount {
private:
    double balance;

public:
    BankAccount(double initial) {
        balance = initial;
    }

    double getBalance() const {
        return balance;
    }

    void deposit(double amount) {
        balance += amount;
    }
};

int main() {
    BankAccount account(100.0);
    account.deposit(50.0);
    cout << "Balance: " << account.getBalance() << endl;
    return 0;
}

Balance: 150

balance can never be set to an invalid value from outside BankAccount, because nothing outside the class can reach it directly — every change has to go through deposit, which the class controls completely.

Next: inheritance and polymorphism — building a class hierarchy, and dispatching to the right override at runtime.