C++: What is the virtual table (vtable) and how does virtual dispatch work?

1class Base {
2public:
3    virtual void foo() { std::cout << "Base\n"; }
4    virtual void bar() { std::cout << "Base\n"; }
5    int x;
6};
7
8class Derived : public Base {
9public:
10    void foo() override { std::cout << "Derived\n"; }
11};
12
13// Memory layout:
14// Base: [vptr | x]
15// Derived: [vptr | x]
16
17// Virtual dispatch:
18Base* p = new Derived();
19p->foo();  // Follows vptr -> Derived vtable -> Derived::foo()
20
21// Cost:
22// - Extra pointer per object (vptr).
23// - Indirect function call.
24// - Cannot be inlined.
25// - Prevents some optimizations.