class Container 
{ 
public:
virtual double& operator[](int) = 0; virtual int size() const = 0;
virtual  ̃Container() {}
// pure virtual function
// const member function (§3.2.1.1) // destructor (§3.2.1.2)
};

void use(Container& c) 
{
const int sz = c.size();
for (int i=0; i!=sz; ++i) cout << c[i] << '\n';
}
 

class Vector_container : public Container 
{ // Vector_container implements Container Vector v;
public:
Vector_container(int s) : v(s) { } // Vector of s elements  ̃Vector_container() {}
double& operator[](int i) { return v[i]; }
int size() const { return v.size(); } };

class List_container : public Container { // List_container implements Container std::list<double> ld; // (standard-library) list of doubles (§4.4.2)
public:
List_container() { } // empty List List_container(initializer_list<double> il) : ld{il} { }  ̃List_container() {}

double& operator[](int i);
int size() const { return ld.size(); }
};
double& List_container::operator[](int i) {
for (auto& x : ld) {
if (i==0) return x;
−−i; }
throw out_of_range("List container"); }
 

void h() {
List_container lc = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
use(lc); }
void g() {
Vector_container vc {10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0};
use(vc); }

g()和h()中的use呼叫給Container &c綁定了不同的物件，他們分別動態繫結到Vector_container和List_container的operator[]成員上

因此，Container物件必包含runtime選擇正確函式的資訊
編譯器將虛擬函式的名字轉換成函式指標表中對應的索引值，這個表稱為虛擬函式表.
空間開銷：每個類有一個vtbl，類的每個物件有一個額外的指標