Classes & Objects

Member functions outside the class

• Declare inside; define outside with scope resolution Class::method.
• Keeps the class body shorter — common in headers + .cpp files.

class Box {
public:
    int volume(int l, int w, int h);
};

int Box::volume(int l, int w, int h) {
    return l * w * h;
}

const member functions and encapsulation habits

• Mark readers const: int getBalance() const; — promises not to modify the object.
• Provide getters/setters instead of public data when rules matter (validation).
• Prefer invariants: withdraw should not allow negative balance.

class Account {
private:
    int balance = 0;
public:
    int getBalance() const { return balance; }
    bool withdraw(int amt) {
        if (amt <= 0 || amt > balance) return false;
        balance -= amt;
        return true;
    }
};

Separating interface and implementation

• Put class declaration in a .h / .hpp; define members in a .cpp.
• Include the header where objects are created — improves compile times and clarity.

Real-life class designs

• Bank: Account with deposit/withdraw/checkBalance.
• School: Student with roll, name, marks, showResult().
• Shop: Product with id, price, stock, applyDiscount().
• Game: Player with health, score, attack(), heal().

Car class with two objects

#include <iostream>
#include <string>
using namespace std;

class Car {
private:
    string brand;
    int speed;
public:
    void setData(string b, int s) { brand = b; speed = s; }
    void showData() {
        cout << brand << " @ " << speed << " km/h\n";
    }
};

int main() {
    Car a, b;
    a.setData("Toyota", 120);
    b.setData("Honda", 100);
    a.showData();
    b.showData();
    return 0;
}

Student class — set and show

#include <iostream>
#include <string>
using namespace std;

class Student {
private:
    int roll;
    string name;
public:
    void set(int r, string n) { roll = r; name = n; }
    void show() const { cout << roll << " " << name << "\n"; }
};

int main() {
    Student s;
    s.set(11, "Neha");
    s.show();
    return 0;
}

struct vs class default access

#include <iostream>
using namespace std;

struct S { int x; };      // public by default
class C { int y; public: void set(int v) { y = v; } int get() const { return y; } };

int main() {
    S s; s.x = 5;         // OK
    C c; c.set(7);        // OK via method
    cout << s.x << " " << c.get() << "\n";
    return 0;
}

Account — deposit and withdraw

#include <iostream>
using namespace std;

class Account {
private:
    int balance = 0;
public:
    void deposit(int amt) { if (amt > 0) balance += amt; }
    bool withdraw(int amt) {
        if (amt <= 0 || amt > balance) return false;
        balance -= amt;
        return true;
    }
    int getBalance() const { return balance; }
};

int main() {
    Account acc;
    acc.deposit(1000);
    acc.withdraw(250);
    cout << acc.getBalance() << "\n"; // 750
    return 0;
}

Array of objects

#include <iostream>
#include <string>
using namespace std;

class Item {
public:
    string name;
    int qty;
    void show() const { cout << name << " x" << qty << "\n"; }
};

int main() {
    Item cart[2] = {{"Pen", 3}, {"Book", 1}};
    for (const Item& it : cart) it.show();
    return 0;
}

Method defined outside class

#include <iostream>
using namespace std;

class Box {
public:
    int volume(int l, int w, int h);
};

int Box::volume(int l, int w, int h) {
    return l * w * h;
}

int main() {
    Box b;
    cout << b.volume(2, 3, 4) << "\n"; // 24
    return 0;
}

Pointer to object

#include <iostream>
using namespace std;

class Counter {
private:
    int n = 0;
public:
    void inc() { n++; }
    int get() const { return n; }
};

int main() {
    Counter c;
    Counter* p = &c;
    p->inc();
    p->inc();
    cout << p->get() << "\n"; // 2
    return 0;
}