Classes — member functions, access specifiers (public, private, protected)
1. What is a class? (WHAT)
class BankAccount {
private: // walls go up here
double balance; // member variable (state)
public:
void deposit(double amount); // member function (behaviour)
double getBalance() const;
};WHY separate the class from the object? One blueprint (class) can stamp out millions of objects, each with its own copy of the member variables but sharing the same member-function code.
2. Access specifiers — the three walls (WHAT + WHY)
| Specifier | Same class | Derived class | Outside (main) |
|---|---|---|---|
| public | ✅ | ✅ | ✅ |
| protected | ✅ | ✅ | ❌ |
| private | ✅ | ❌ | ❌ |
3. Member functions (HOW)
Defining inside vs outside the class
class BankAccount {
private:
double balance = 0;
public:
// (a) defined INSIDE → implicitly inline
double getBalance() const { return balance; }
// (b) declared inside, DEFINED OUTSIDE with scope resolution ::
void deposit(double amount);
};
void BankAccount::deposit(double amount) { // :: ties it to the class
balance += amount; // 'balance' really means this->balance
}4. Full worked example (Derivation-from-scratch)
#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
}Why STEP 1? If width/height were public, anyone could set a negative width → an impossible rectangle.
Why STEP 2 validates? The only door into the data also acts as a guard — invalid states become unrepresentable.
Why area() is const? Computing area doesn't change the rectangle; marking it const documents and enforces that.
5. Common mistakes (Steel-man + fix)
6. The 80/20 core
Recall Feynman: explain to a 12-year-old
Imagine a vending machine. Inside it there's money and snacks (the private data) — you can't reach in and grab them. On the outside there are buttons (the public functions): "insert coin", "press B4". You only interact through the buttons, and the machine makes sure you can't steal or break things. A class is the design of one vending machine; each actual machine in the hallway is an object. Private = stuff locked inside the box. Public = the buttons. Protected = a secret repair panel only the machine's family of repair-machines can open, but not customers.
Connections
- Encapsulation & Data Hiding
- Constructors and Destructors
- Inheritance in C++ — where
protectedbecomes essential - const correctness
- friend functions and classes
- this pointer
- struct vs class
Flashcards
What are the three access specifiers in C++?
What is the default access level in a class?
What is the default access level in a struct?
Where can a private member be accessed?
What does protected add over private?
Access in C++ is per-class or per-object?
What does the hidden this pointer point to?
What does const after a member function signature guarantee?
What operator ties an externally-defined function to its class?
::Why hide data behind public functions?
Can you call a non-const member function on a const object?
Concept Map
Hinglish (regional understanding)
Intuition Hinglish mein samjho
Dekho, ek class ka matlab hai ek blueprint — jaise vending machine ka design. Usme do cheezein hoti hain: data (member variables, jaise balance) aur functions (member functions, jaise deposit()). In dono ko ek saath bundle karna hi encapsulation kehlata hai. Asli faayda ye hai ki hum data ko bahar wale logon se chhupa sakte hain, taaki koi galat tareeke se balance = -9999 na kar de.
Ab access specifiers wo deewaarein hain jo decide karti hain kaun kya chhoo sakta hai. private matlab sirf apni class ke andar — bilkul locked. public matlab sabke liye open, jaise vending machine ke buttons. Aur protected ek beech ka level hai: apni class plus bachche (derived classes) use kar sakte hain, lekin bahar wala main() nahi. Yaad rakho — class me by default sab kuch private hota hai, jabki struct me public. Bas yahi ek difference hai dono me.
Member functions ke paas ek chhupa hua this pointer hota hai jo bata raha hai "main kis object pe kaam kar raha hoon." Agar function object ko change nahi karta, to uske aage const likho — ye ek promise hai compiler ko, aur compiler isse enforce bhi karta hai. Class ke bahar function define karne ke liye ReturnType ClassName::function() likhte hain, jisme :: batata hai ki ye function kis class ka hai.
80/20 baat ye hai: data ko private rakho, usse access karne ke liye public functions (gateways) banao jo validation bhi karein. Isse "invalid state" possible hi nahi rahega — yahi professional C++ code ka core hai. Exam aur interview dono me ye table (kaun-kaun access kar sakta hai) zaroor poocha jaata hai, to ratta nahi, samajh ke yaad karo.