What are virtual functions and vtables?

Question

Accepted Answer

**Virtual functions** — runtime polymorphism.

```cpp
class Shape {
public:
    virtual double area() const = 0; // pure virtual
    virtual ~Shape() = default;
};

class Circle : public Shape {
    double r;
public:
    double area() const override { return 3.14 * r * r; }
};
```

**Vtable mechanism:**
- Each class with virtual functions has a vtable
- vtable contains function pointers
- Objects have hidden vptr to their class vtable
- Virtual call: vptr -> vtable -> function

**Cost:**
- Extra pointer per object (8 bytes)
- Indirect call (cache miss possible)
- Prevents inlining

**When to use:**
- Polymorphic interfaces
- When you need runtime type selection
- Avoid for simple performance-sensitive code

C++: What are virtual functions and vtables?