Inheritance and virtual functions in C++

Output questions on inheritance usually turn on a single keyword: virtual. Knowing exactly what it changes, along with how overloading and const work, lets you answer these questions with confidence.

Static versus dynamic binding

When you call a member function through a base pointer or reference, a non-virtual function is chosen from the declared type at compile time. A virtual function is chosen from the actual object's type at runtime.

class Shape {
public:
    virtual void draw() const { cout << "Shape\n"; }
    void name() const { cout << "shape\n"; }
    virtual ~Shape() = default;
};
class Circle : public Shape {
public:
    void draw() const override { cout << "Circle\n"; }
    void name() const { cout << "circle\n"; }
};

Shape* s = new Circle();
s->draw();   // Circle (virtual)
s->name();   // shape  (not virtual)
delete s;

Write override on every overriding function. If the signature does not match, for example because const is missing, the compiler reports an error instead of silently creating a new function.

Virtual destructors and abstract classes

Deleting a derived object through a base pointer when the base destructor is not virtual is undefined behaviour, and in practice the derived destructor usually never runs. Give any class meant for polymorphic use a virtual destructor. A class with at least one pure virtual function, written = 0, is abstract: you cannot create objects of it, only of derived classes that implement every pure virtual function.

Overloading and const

Overloaded functions share a name but differ in their parameters. The compiler picks the best match, preferring an exact match, then a promotion such as char to int, then a conversion such as int to double.

void print(int)    { cout << "int\n"; }
void print(double) { cout << "double\n"; }
print('x');    // int: char is promoted to int
print(2.5f);   // double: float is promoted to double

A const member function promises not to change the object. Only const member functions can be called on a const object or through a const reference.

Checklist for inheritance snippets

1. Is the function virtual in the base class?
2. What is the pointer type, and what is the object type?
3. Does the derived signature match exactly, including const?
4. Is the destructor virtual if the object is deleted through a base pointer?