const Correctness

Marking a member function const (seen already on getBalance() and area() in earlier lessons) promises it won't modify the object — the compiler enforces this, and only a const method can be called on a const object or reference.

class Point {
private:
    int x, y;

public:
    Point(int x, int y) : x(x), y(y) {}

    int getX() const { return x; }  // doesn't modify — const
    void setX(int newX) { x = newX; } // modifies — not const
};

void printPoint(const Point &p) {
    std::cout << p.getX() << std::endl; // fine — getX is const
    // p.setX(5);  // would NOT compile — p is a const reference
}

Passing large objects as const & (rather than by value) is the default in idiomatic C++ — no copy is made, and const guarantees the function can't accidentally modify the caller's object.

Common mistakes

• Passing an object by value out of habit when const & would avoid an unnecessary copy — for anything larger than a couple of primitives, prefer const &.