虛擬函式的作用：用於實現C++的多型。
虛擬函式表：具有虛擬函式的類在編譯階段會建立一個虛擬函式表vtable。
虛擬函式指標：每個類物件有一個虛擬函式指標vptr，vptr指向vtable。

虛擬函式表是虛擬函式指標的陣列
多重繼承的虛擬函式表：
在派生類Derived虛擬函式表中，派生類的虛擬函式f()會覆蓋所有基類的f()
同時，派生類的其它虛擬函式g1()會放在第一個基類的虛擬函式表後面

class Base1 {
public:
    Base1() { cout << "Base1::Base1()" << endl; }
    virtual void f() { cout << "Base1::f()" << endl; }
    virtual void g() { cout << "Base1::g()" << endl; }
    virtual void h() { cout << "Base1::h()" << endl; }
};

class Base2 {
public:
    Base2() { cout << "Base2::Base2()" << endl; }
    virtual void f() { cout << "Base2::f()" << endl; }
    virtual void g() { cout << "Base2::g()" << endl; }
    virtual void h() { cout << "Base2::h()" << endl; }
};

class Derived : public Base1, public Base2 {
public:
    Derived() { cout << "Derived::Derived()" << endl; }
    void f() { cout << "Derived::f()" << endl; }
    virtual void g1() { cout << "Derived::g1()" << endl; }
};

void printDerived(Derived &d)
{
    cout << endl << "print all vtable:" << endl;
    int i = 0, j = 0;
    typedef void(*Fcn)();

    while (i < 2) {
        Fcn vfcn = (Fcn)*((int*)(*((int*)&d + i)) + j);
        cout << "Base" << i + 1 << "::vtable:" << endl;
        while (vfcn) {
            vfcn();
            ++j;
            vfcn = (Fcn)*((int*)*((int*)&d + i) + j);
        }
        ++i;
        j = 0;
    }
}

void test()
{
    Derived d;
    printDerived(d);
}
 

win32下：
Base1虛擬函式表的地址：(int*)&d + 0
Base2虛擬函式表的地址：(int*)&d + 1
Base1虛擬函式表第1個虛擬函式指標的地址：(int*)*((int)&d + 0) + 0

Base1虛擬函式表第2個虛擬函式指標的地址：(int*)*((int)&d + 0) + 1

Base1虛擬函式表第3個虛擬函式指標的地址：(int*)*((int)&d + 0) + 2

Base2虛擬函式表第1個虛擬函式指標的地址：(int*)*((int)&d + 1) + 0

Base2虛擬函式表第2個虛擬函式指標的地址：(int*)*((int)&d + 1) + 1

Base2虛擬函式表第3個虛擬函式指標的地址：(int*)*((int)&d + 1) + 2

多重繼承虛擬函式表示意圖：