多態&多態對象模型

1：什麼是多態？
多態就是多種形態。
滿足構成多態的條件：必須有虛函數的重寫（函數名，參數，返回值均相同，協變除外）
父類的指針或引用調用虛函數
多態跟類型無關而和對象有關，如果不構成多態則與類型有關
父類必須爲虛函數，子類可以是也可以不是，構造函數不能爲虛函數。

class A
{
public:
    virtual void func()//父類必須爲虛函數
    {
        cout << "A::func()" << endl;
    }
protected:
    int _a;
};
class B:public A
{
public:
    virtual void func()
    {
        cout << "B::func()" << endl;
    }
protected:
    int _b;
};
//void f(A* a)
void f(A& a)//父類的指針或引用調用虛函數
{
    a.func();
}

int main()
{
    A a;
    B b;
    f(a);
    f(b);
    A* p1 = &a;
    p1->func();
    p1 = &b;
    p1->func();
    system("pause");
    return 0;
}

如果A中沒有virtual，則以上都調用父類

2：多態的對象模型–單繼承&多繼承？
虛函數表是通過一塊連續內存來存儲虛函數的地址。解決了繼承，虛函數（重寫）的問題，指明瞭實際調用虛函數的問題。

#include<iostream>
using namespace std;

typedef void(*V_FUNC)();
class A
{
public:
    virtual void func1()
    {
        cout << "A::func1()" << endl;
    }
    virtual void func2()
    {
        cout << "A::func2()" << endl;
    }
protected:
    int a;
};
class B:public A
{
public:
    virtual void func1()
    {
        cout << "B::func1()" << endl;
    }
    virtual void func3()
    {
        cout << "B::func3()" << endl;
    }
    virtual void func4()
    {
        cout << "B::func4()" << endl;
    }
protected:
    int b;
};
void PrintVtable(int vtable)//打印虛表
{
    int *vfArray = (int*)vtable;
    printf("vtable:0x%p\n", vfArray);
    for (size_t i = 0; vfArray[i] != 0; ++i)//虛表以0結束
    {
        printf("vfunc[%d]:0x%p->", i, vfArray[i]);
        V_FUNC f = (V_FUNC)vfArray[i];
        f();
    }
    cout << endl;
}

int main()
{
    A a;
    B b;

    PrintVtable(*((int*)&a));
    PrintVtable(*((int*)&b));

    system("pause");
    return 0;

}

由於編譯器的優化，在監視窗口中關於B調用的虛函數並不能顯示完全，所以需要我們自己寫一個打印虛表的函數

以上是單繼承的模型
多繼承中子集將虛函數存放在第一個繼承的虛函數表中

#include<iostream>
using namespace std;

typedef void(*V_FUNC)();
class A
{
public:
    virtual void func1()
    {
        cout << "A::func1()" << endl;
    }
    virtual void func2()
    {
        cout << "A::func2()" << endl;
    }
protected:
    int a;
};
class B
{
public:
    virtual void func1()
    {
        cout << "B::func1()" << endl;
    }
    virtual void func3()
    {
        cout << "B::func3()" << endl;
    }
protected:
    int b;
};
class C:public A,public B
{
public:
    virtual void func1()
    {
        cout << "C::func1()" << endl;
    }
    virtual void func3()
    {
        cout << "C::func3()" << endl;
    }
    virtual void func4()
    {
        cout << "C::func4()" << endl;
    }
protected:
    int c;
};
void PrintVtable(int vtable)
{
    int *vfArray = (int*)vtable;
    printf("vtable:0x%p\n", vfArray);
    for (size_t i = 0; vfArray[i] != 0; ++i)
    {
        printf("vfunc[%d]:0x%p->", i, vfArray[i]);
        V_FUNC f = (V_FUNC)vfArray[i];
        f();
    }
    cout << endl;
}

int main()
{
    C c;

    PrintVtable(*((int*)&c));//A的虛表
    PrintVtable(*(int*)((char*)&c + sizeof(A)));//B的虛表在A的虛表之後，相差sizeof（A）



    system("pause");
    return 0;

}

如func1（），A和C中都有，則進行虛函數重定義

3：多態的對象模型–菱形繼承和菱形虛擬繼承？

#include<iostream>
using namespace std;

typedef void(*V_FUNC)();
class A
{
public:
    virtual void func1()
    {
        cout << "A::func1()" << endl;
    }
    virtual void func2()
    {
        cout << "A::func2()" << endl;
    }
protected:
    int a;
};
class B:public A
{
public:
    virtual void func1()
    {
        cout << "B::func1()" << endl;
    }
    virtual void func3()
    {
        cout << "B::func3()" << endl;
    }
protected:
    int b;
};
class C:public A
{
public:
    virtual void func1()
    {
        cout << "C::func1()" << endl;
    }
    virtual void func4()
    {
        cout << "C::func4()" << endl;
    }
protected:
    int c;
};
class D :public B, public C
{
public:
    virtual void func5()
    {
        cout << "D::func5()" << endl;
    }
protected:
    int d;
};
void PrintVtable(int vtable)
{
    int *vfArray = (int*)vtable;
    printf("vtable:0x%p\n", vfArray);
    for (size_t i = 0; vfArray[i] != 0; ++i)
    {
        printf("vfunc[%d]:0x%p->", i, vfArray[i]);
        V_FUNC f = (V_FUNC)vfArray[i];
        f();
    }
    cout << endl;
}

int main()
{
    D d;

    PrintVtable(*((int*)&d));
    PrintVtable(*(int*)((char*)&d + sizeof(B)));


    system("pause");
    return 0;

}

菱形虛擬繼承

 class A
{
public:
    virtual void func1()
    {
        cout << "A::func1()" << endl;
    }
    virtual void func2()
    {
        cout << "A::func2()" << endl;
    }

public:
    int _a;
};

class B : virtual public A
{
public:
    virtual void func1()
    {
        cout << "B::func1()" << endl;
    }
    virtual void func3()
    {
        cout << "B::func3()" << endl;
    }

public:
    int _b;
};

class C : virtual public A 
{
    virtual void func1()
    {
        cout << "C::func1()" << endl;
    }

    virtual void func3()
    {
        cout << "C::func3()" << endl;
    }

public:
    int _c;
};

class D : public B, public C
{
    virtual void func1()
    {
        cout << "D::func1()" << endl;
    }

    virtual void func4()
    {
        cout << "D::func4()" << endl;
    }

public:
    int _d;
}; 

菱形虛擬繼承（B，C共用A的虛表，各自有獨立的虛表）