【C++】面向对象编程的三大特性：深入解析多态机制（二）

class Base
{
    public:
    virtual void Func1()
    {
        cout << "Func1()" << endl;
    }
    private:
    int _b = 1;
};
结果:sizeof(Base) == 8

class Base
{
    public:
    virtual void Func1()
    {
        cout << "Base::Func1()" << endl;
    }
    virtual void Func2()
    {
        cout << "Base::Func2()" << endl;
    }
    //非虚函数
    void Func3()
    {
        cout << "Base::Func3()" << endl;
    }
    private:
    int _b = 1;
};
class Derive : public Base
{
    public:
    //虚函数func1的重写
    virtual void Func1()
    {
        cout << "Derive::Func1()" << endl;
    }
    private:
    int _d = 2;
};
int main()
{
    Base b;
    Derive d;
    return 0;
}

class Person
{
    public:
    virtual void BuyTicket() { cout << "买票-全价" << endl; }
};
class Student : public Person
{
    public:
    virtual void BuyTicket() { cout << "买票-半价" << endl; }
};
void Func(Person& p)
{
    p.BuyTicket();
}
int main()
{
    Person Mike;
    Func(Mike);
    Student Johnson;
    Func(Johnson);
    return 0;
}

void Func(Person* p)
{
    ...
        p->BuyTicket();
    // p中存的是mike对象的指针，将p移动到eax中
    001940DE mov eax,dword ptr [p]
        // [eax]就是取eax值指向的内容，这里相当于把mike对象头4个字节(虚表指针)移动到了edx
        001940E1 mov edx,dword ptr [eax]
        // [edx]就是取edx值指向的内容，这里相当于把虚表中的头4字节存的虚函数指针移动到了eax
        00B823EE mov eax,dword ptr [edx]
        // call eax中存虚函数的指针。这里可以看出满足多态的调用，不是在编译时确定的，是运行起来以后到对象的中取找的。
        001940EA call eax
        00头1940EC cmp esi,esp
}
int main()
{
    ...
        // 首先BuyTicket虽然是虚函数，但是mike是对象，不满足多态的条件，所以这里是普通函数的调用转换成地址时，是在编译时已经从符号表确认了函数的地址，直接call 地址
        mike.BuyTicket();
    00195182 lea ecx,[mike]
        00195185 call Person::BuyTicket (01914F6h)
        ...
}

int main()
{
    int i = 0;
    static int j = 1;
    int* p1 = new int;
    const char* p2 = "xxxxxxxx";
    printf("栈:%p\n", &i);
    printf("静态区:%p\n", &j);
    printf("堆:%p\n", p1);
    printf("常量区:%p\n", p2);
    Person p;
    Student s;
    Person* p3 = &p;
    Student* p4 = &s;
    printf("Person虚表地址:%p\n", *(int*)p3);
    printf("Student虚表地址:%p\n", *(int*)p4);
    return 0;
}