重複繼承
下面我們再來看看，發生重複繼承的情況。所謂重複繼承，也就是某個基類被間接地重複繼承了多次。
下圖是一個繼承圖，我們重載了父類的f()函數。
其類繼承的源代碼如下所示。其中，每個類都有兩個變量，一個是整形（4字節），一個是字符（1字節），而且還有自己的虛函數，自己 overwrite父類的虛函數。如子類D中，f()覆蓋了超類的函數， f1() 和f2() 覆蓋了其父類的虛函數，Df()爲自己的虛函數。
class B
{
public:
int ib;
char cb;
public:
B():ib(0),cb('B') {}
virtual void f() { cout << "B::f()" << endl;}
virtual void Bf() { cout << "B::Bf()" << endl;}
};
class B1 : public B
{
public:
int ib1;
char cb1;
public:
B1():ib1(11),cb1('1') {}
virtual void f() { cout << "B1::f()" << endl;}
virtual void f1() { cout << "B1::f1()" << endl;}
virtual void Bf1() { cout << "B1::Bf1()" << endl;}
};
class B2: public B
{
public:
int ib2;
char cb2;
public:
B2():ib2(12),cb2('2') {}
virtual void f() { cout << "B2::f()" << endl;}
virtual void f2() { cout << "B2::f2()" << endl;}
virtual void Bf2() { cout << "B2::Bf2()" << endl;}
};
class D : public B1, public B2
{
public:
int id;
char cd;
public:
D():id(100),cd('D') {}
virtual void f() { cout << "D::f()" << endl;}
virtual void f1() { cout << "D::f1()" << endl;}
virtual void f2() { cout << "D::f2()" << endl;}
virtual void Df() { cout << "D::Df()" << endl;}
};
我們用來存取子類內存佈局的代碼如下所示：（在VC++ 2003和G++ 3.4.4下）
typedef void(*Fun)(void);
int** pVtab = NULL;
Fun pFun = NULL;
D d;
pVtab = (int**)&d;
cout << "[0] D::B1::_vptr->" << endl;
pFun = (Fun)pVtab[0][0];
cout << " [0] "; pFun();
pFun = (Fun)pVtab[0][1];
cout << " [1] "; pFun();
pFun = (Fun)pVtab[0][2];
cout << " [2] "; pFun();
pFun = (Fun)pVtab[0][3];
cout << " [3] "; pFun();
pFun = (Fun)pVtab[0][4];
cout << " [4] "; pFun();
pFun = (Fun)pVtab[0][5];
cout << " [5] 0x" << pFun << endl;
cout << "[1] B::ib = " << (int)pVtab[1] << endl;
cout << "[2] B::cb = " << (char)pVtab[2] << endl;
cout << "[3] B1::ib1 = " << (int)pVtab[3] << endl;
cout << "[4] B1::cb1 = " << (char)pVtab[4] << endl;
cout << "[5] D::B2::_vptr->" << endl;
pFun = (Fun)pVtab[5][0];
cout << " [0] "; pFun();
pFun = (Fun)pVtab[5][1];
cout << " [1] "; pFun();
pFun = (Fun)pVtab[5][2];
cout << " [2] "; pFun();
pFun = (Fun)pVtab[5][3];
cout << " [3] "; pFun();
pFun = (Fun)pVtab[5][4];
cout << " [4] 0x" << pFun << endl;
cout << "[6] B::ib = " << (int)pVtab[6] << endl;
cout << "[7] B::cb = " << (char)pVtab[7] << endl;
cout << "[8] B2::ib2 = " << (int)pVtab[8] << endl;
cout << "[9] B2::cb2 = " << (char)pVtab[9] << endl;
cout << "[10] D::id = " << (int)pVtab[10] << endl;
cout << "[11] D::cd = " << (char)pVtab[11] << endl;
程序運行結果如下：
|
GCC 3.4.4 |
VC++ 2003 |
|
[0] D::B1::_vptr-> [0] D::f() [1] B::Bf() [2] D::f1() [3] B1::Bf1() [4] D::f2() [5] 0x1 [1] B::ib = 0 [2] B::cb = B [3] B1::ib1 = 11 [4] B1::cb1 = 1 [5] D::B2::_vptr-> [0] D::f() [1] B::Bf() [2] D::f2() [3] B2::Bf2() [4] 0x0 [6] B::ib = 0 [7] B::cb = B [8] B2::ib2 = 12 [9] B2::cb2 = 2 [10] D::id = 100 [11] D::cd = D |
[0] D::B1::_vptr-> [0] D::f() [1] B::Bf() [2] D::f1() [3] B1::Bf1() [4] D::Df() [5] 0x00000000 [1] B::ib = 0 [2] B::cb = B [3] B1::ib1 = 11 [4] B1::cb1 = 1 [5] D::B2::_vptr-> [0] D::f() [1] B::Bf() [2] D::f2() [3] B2::Bf2() [4] 0x00000000 [6] B::ib = 0 [7] B::cb = B [8] B2::ib2 = 12 [9] B2::cb2 = 2 [10] D::id = 100 [11] D::cd = D |
下面是對於子類實例中的虛函數表的圖：
我們可以看見，最頂端的父類B其成員變量存在於B1和B2中，並被D給繼承下去了。而在D中，其有B1和B2的實例，於是B的成員在D的實例中存在兩份，一份是B1繼承而來的，另一份是B2繼承而來的。所以，如果我們使用以下語句，則會產生二義性編譯錯誤：
D d;
d.ib = 0; //二義性錯誤
d.B1::ib = 1; //正確
d.B2::ib = 2; //正確
注意，上面例程中的最後兩條語句存取的是兩個變量。雖然我們消除了二義性的編譯錯誤，但B類在D中還是有兩個實例，這種繼承造成了數據的重複，我們叫這種繼承爲重複繼承。重複的基類數據成員可能並不是我們想要的。所以，C++引入了虛基類的概念。