Classes — member functions, access specifiers (public, private, protected)

class BankAccount {
private:                 // walls go up here
    double balance;      // member variable (state)
public:
    void deposit(double amount);   // member function (behaviour)
    double getBalance() const;
};

class BankAccount {
private:
    double balance = 0;
public:
    // (a) INSIDE define kiya → implicitly inline
    double getBalance() const { return balance; }
 
    // (b) andar declare, OUTSIDE define kiya scope resolution :: ke saath
    void deposit(double amount);
};
 
void BankAccount::deposit(double amount) {   // :: ties it to the class
    balance += amount;        // 'balance' really means this->balance
}

#include <iostream>
using namespace std;
 
class Rectangle {
private:                              // STEP 1: hide raw data
    double width, height;
public:
    // STEP 2: controlled way to set state (validates input!)
    void setDimensions(double w, double h) {
        if (w < 0 || h < 0) { width = height = 0; return; }
        width = w; height = h;
    }
    // STEP 3: read-only computed property
    double area() const { return width * height; }
};
 
int main() {
    Rectangle r;
    r.setDimensions(3, 4);            // ✅ public gateway
    cout << r.area();                 // prints 12
    // r.width = -5;                  // ❌ blocked: cannot create invalid state
}