一. 多態(tài)的實(shí)現(xiàn)--虛表
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1.多態(tài)就是多種形態(tài)。在C++中，對(duì)同一父類和子類構(gòu)造出的不同對(duì)象發(fā)出同一條指令，需要它們完成不同的工作，就需要在父類和子類寫出相同函數(shù)名的函數(shù)方法，并用虛函數(shù)區(qū)分它們。父類的指針/引用調(diào)用重寫的虛函數(shù)，當(dāng)父類指針/引用指向父類對(duì)象時(shí)調(diào)用的是父類的虛函數(shù)，指向子類對(duì)象時(shí)調(diào)用的是子類的虛函數(shù)。
2.虛函數(shù)表是通過(guò)一塊連續(xù)內(nèi)存來(lái)存儲(chǔ)虛函數(shù)的地址。這張表解決了繼承、虛函數(shù)(重寫)的問(wèn)題。在有虛函數(shù)的對(duì)象實(shí)例中都存在一張?zhí)摵瘮?shù)表，虛函數(shù)表就像一張地圖，指明了實(shí)際應(yīng)該調(diào)用的虛函數(shù)函數(shù)。
重慶分公司
重慶分公司
//帶虛函數(shù)的類
class Base
{
public:
virtual void func1()
{
cout << "Base::func1()" << endl;
}
virtual void func2()
{
cout << "Base::func2()" << endl;
}
private:
int a;
};
void Test1()
{
Base b1;
}3.帶虛函數(shù)的類的模型結(jié)構(gòu)
4.同一類的對(duì)象共用同一虛表
void Test2()
{
Base b1, b2, b3;
}
二. 單繼承&多繼承的對(duì)象模型
1.單繼承：
class Base
{
public:
virtual void func1()
{
cout << "Base::func1()" << endl;
}
virtual void func2()
{
cout << "Base::func2()" << endl;
}
private:
int a;
};
class Derive :public Base
{
public:
virtual void func1()
{
cout << "Derive::func1" << endl;
}
virtual void func3()
{
cout << "Derive::func3" << endl;
}
virtual void func4()
{
cout << "Derive::func4" << endl;
}
private:
int b;
};
typedef void(*FUNC) ();
void PrintVTable(int* VTable)
{
cout << " 虛表地址>" << VTable << endl;
for (int i = 0; VTable[i] != 0; ++i)
{
printf(" 第%d個(gè)虛函數(shù)地址 :0X%x,->", i, VTable[i]);
FUNC f = (FUNC)VTable[i];
f();
}
cout << endl;
}
void Test1()
{
Base b1;
Derive d1;
int* VTable1 = (int*)(*(int*)&b1);
int* VTable2 = (int*)(*(int*)&d1);
PrintVTable(VTable1);
PrintVTable(VTable2);
}
2.多繼承
class Base1
{
public:
virtual void func1()
{
cout << "Base1::func1" << endl;
}
virtual void func2()
{
cout << "Base1::func2" << endl;
}
private:
int b1;
};
class Base2
{
public:
virtual void func1()
{
cout << "Base2::func1" << endl;
}
virtual void func2()
{
cout << "Base2::func2" << endl;
}
private:
int b2;
};
class Derive : public Base1, public Base2
{
public:
virtual void func1()
{
cout << "Derive::func1" << endl;
}
virtual void func3()
{
cout << "Derive::func3" << endl;
}
private:
int d1;
};
typedef void(*FUNC) ();
void PrintVTable(int* VTable)
{
cout << " 虛表地址>" << VTable << endl;
for (int i = 0; VTable[i] != 0; ++i)
{
printf(" 第%d個(gè)虛函數(shù)地址 :0X%x,->", i, VTable[i]);
FUNC f = (FUNC)VTable[i];
f();
}
cout << endl;
}
void Test1()
{
Derive d1;
int* VTable = (int*)(*(int*)&d1);
PrintVTable(VTable);
// Base2虛函數(shù)表在對(duì)象Base1后面
VTable = (int *)(*((int*)&d1 + sizeof(Base1) / 4));
PrintVTable(VTable);
}
三. 靜態(tài)多態(tài)和動(dòng)態(tài)多態(tài)
1.靜態(tài)多態(tài)就是重載，因?yàn)槭窃诰幾g期決議確定，所以稱為靜態(tài)多態(tài)。
2.動(dòng)態(tài)多態(tài)就是通過(guò)繼承重寫基類的虛函數(shù)實(shí)現(xiàn)的多態(tài)，因?yàn)槭窃谶\(yùn)行時(shí)決議確定，所以稱為動(dòng)態(tài)多態(tài)。
class Base
{
public:
virtual void func1()
{
cout << "Base::func1" << endl;
}
virtual void func2()
{
cout << "Base::func2" << endl;
}
void display()
{
cout << "display()" << endl;
}
void display(int i)
{
cout << "display(int i)->" << i << endl;
}
private:
int a;
};
class Derive :public Base
{
public:
virtual void func1()
{
cout << "Derive::func1" << endl;
}
virtual void func3()
{
cout << "Derive::func3" << endl;
}
virtual void func4()
{
cout << "Derive::func4" << endl;
}
private:
int b;
};
void func(Base& b)
{
b.func1();
b.display();
b.display(10);
}
void Test1()
{
Base b1;
Derive d1;
func(b1);
func(d1);
}
四. 菱形虛擬繼承的對(duì)象模型
class A
{
public:
virtual void FunTest1()
{
cout << "A::FunTest1()" << endl;
}
virtual void FunTest2()
{
cout << "A::FunTest2()" << endl;
}
private:
int _a;
};
class B1 :virtual public A
{
public:
virtual void FunTest3()
{
cout << "B1::FunTest3()" << endl;
}
virtual void FunTest4()
{
cout << "B2::FunTest4()" << endl;
}
private:
int _b1;
};
class B2 :virtual public A
{
public:
virtual void FunTest1()
{
cout << "B2::FunTest1()" << endl;
}
virtual void FunTest4()
{
cout << "B2::FunTest4()" << endl;
}
private:
int _b2;
};
class C :public B1, public B2
{
public:
virtual void FunTest1()
{
cout << "C::FunTest1()" << endl;
}
virtual void FunTest5()
{
cout << "C::FunTest5()" << endl;
}
private:
int _c;
};
void Test1()
{
C c;
c.FunTest5();
}
可見(jiàn)，在虛擬繼承中，無(wú)論是幾重繼承，最終子類的虛函數(shù)表總是與繼承列表的第一個(gè)父類的虛函數(shù)表合并在一起，但是有覆蓋的虛函數(shù)，自然去覆蓋父類的同名虛函數(shù)，并合并在該父類的虛表中。
