C++虛函數(shù)與虛函數(shù)表

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多態(tài)性可分為兩類：靜態(tài)多態(tài)和動態(tài)多態(tài)。函數(shù)重載和運算符重載實現(xiàn)的多態(tài)屬于靜態(tài)多態(tài)，動態(tài)多態(tài)性是通過虛函數(shù)實現(xiàn)的。

每個含有虛函數(shù)的類有一張?zhí)摵瘮?shù)表(vtbl)，表中每一項是一個虛函數(shù)的地址， 也就是說，虛函數(shù)表的每一項是一個虛函數(shù)的指針。

沒有虛函數(shù)的C++類，是不會有虛函數(shù)表的。

兩張圖：

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簡單例子：

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?1?#include2?#include3?
?4?using?namespace?std;
?5?
?6?class?base
?7?{
?8?????virtual?void?f(){cout<<"base::f"<<endl;};
?9?????virtual?void?g(){cout<<"base::g"<<endl;};
10?????virtual?void?h(){cout<<"base::h"<<endl;};
11?};
12?
13?typedef?void?(*pfun)();
14?
15?void?main()
16?{
17?????DWORD?w=0x4011e0;?//虛函數(shù)表第一項的內(nèi)容，也就是第一個虛函數(shù)的地址
18?
19?????pfun?fun=NULL;
20?????base?b;
21?????base?*pbase=&b;
22?
23?????fun=(pfun)w;
24?????fun();????//調(diào)用第一個虛函數(shù)
25?}

查看對象b在內(nèi)存中：

查看虛函數(shù)表：

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?虛函數(shù)表的指針4個字節(jié)大小（vptr），存在于對象實例中最前面的位置（這是為了保證取到虛函數(shù)表的有最高的性能——如果有多層繼承或是多重繼承的情況下）。這意味著我們通過對象實例的地址得到這張?zhí)摵瘮?shù)表，然后就可以遍歷其中函數(shù)指針，并調(diào)用相應(yīng)的函數(shù)。

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虛函數(shù)表的結(jié)束標(biāo)志在不同的編譯器下是不同的。在VC6.0下，這個值是NULL，如圖：

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另一個例子：

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?1?#include2?
?3?using?namespace?std;
?4?
?5?class?base
?6?{
?7?????virtual?void?f(){cout<<"base::f"<<endl;};
?8?????virtual?void?g(){cout<<"base::g"<<endl;};
?9?????virtual?void?h(){cout<<"base::h"<<endl;};
10?};
11?
12?class?Derive?:?public?base
13?{
14?????
15?public:
16?????Derive(){};
17?????virtual?void?f()?{?cout?<<?"Derive::f"?<<?endl;?}????
18?????virtual?void?g()?{?cout?<<?"Derive::g"?<<?endl;?}
19?????
20?};
21?
22?typedef?void(*pfun)();
23?
24?void?main()
25?{
26?????pfun?fun=NULL;
27?????Derive?d;
28?????base?*p=&d;
29?
30?????fun=(pfun)**((int**)p);
31?????fun();????//調(diào)用第一個虛函數(shù)
32?
33?????fun=(pfun)*(*((int**)p)+2);
34?????fun();????//調(diào)用第三個函數(shù)
35?
36?}

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查看對象d在內(nèi)存中：

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多重繼承：

?? 有幾個父類，就有幾個vtab和vptr

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代碼：

??1?#include2??
??3?using?namespace?std;
??4??
??5?class?Base1?{
??6??
??7?public:
??8??
??9?????????????virtual?void?f()?{?cout?<<?"Base1::f"?<<?endl;?}
?10??
?11?????????????virtual?void?g()?{?cout?<<?"Base1::g"?<<?endl;?}
?12??
?13?????????????virtual?void?h()?{?cout?<<?"Base1::h"?<<?endl;?}
?14??
?15??
?16??
?17?};
?18??
?19?class?Base2?{
?20??
?21?public:
?22??
?23?????????????virtual?void?f()?{?cout?<<?"Base2::f"?<<?endl;?}
?24??
?25?????????????virtual?void?g()?{?cout?<<?"Base2::g"?<<?endl;?}
?26??
?27?????????????virtual?void?h()?{?cout?<<?"Base2::h"?<<?endl;?}
?28??
?29?};
?30??
?31??
?32?class?Base3?{
?33??
?34?public:
?35??
?36?????????????virtual?void?f()?{?cout?<<?"Base3::f"?<<?endl;?}
?37??
?38?????????????virtual?void?g()?{?cout?<<?"Base3::g"?<<?endl;?}
?39??
?40?????????????virtual?void?h()?{?cout?<<?"Base3::h"?<<?endl;?}
?41??
?42?};
?43??
?44??
?45?class?Derive?:?public?Base1,?public?Base2,?public?Base3?{
?46??
?47?public:
?48??
?49?????????????virtual?void?f()?{?cout?<<?"Derive::f"?<<?endl;?}
?50??
?51?????????????virtual?void?g1()?{?cout?<<?"Derive::g1"?<<?endl;?}
?52??
?53?};
?54??
?55??
?56?typedef?void(*Fun)(void);
?57??
?58?int?main()?
?59?
?60?{
?61??
?62?????????????Fun?pFun?=?NULL;
?63??
?64?????????????Derive?d;
?65??
?66?????????????int**?pVtab?=?(int**)&d;
?67??
?68?????????????//Base1's?vtable
?69??
?70?????????????//pFun?=?(Fun)*((int*)*(int*)((int*)&d+0)+0);
?71??
?72?????????????pFun?=?(Fun)pVtab[0][0];
?73??
?74?????????????pFun();
?75??
?76?
?77?????????????//pFun?=?(Fun)*((int*)*(int*)((int*)&d+0)+1);
?78??
?79?????????????pFun?=?(Fun)pVtab[0][1];
?80??
?81?????????????pFun();
?82??
?83?
?84?????????????//pFun?=?(Fun)*((int*)*(int*)((int*)&d+0)+2);
?85??
?86?????????????pFun?=?(Fun)pVtab[0][2];
?87??
?88?????????????pFun();
?89??
?90?
?91?????????????//Derive's?vtable
?92??
?93?????????????//pFun?=?(Fun)*((int*)*(int*)((int*)&d+0)+3);
?94??
?95?????????????pFun?=?(Fun)pVtab[0][3];
?96??
?97?????????????pFun();
?98??
?99??
100?????????????//The?tail?of?the?vtable
101??
102?????????????pFun?=?(Fun)pVtab[0][4];
103??
104?????????????cout<<pFun<<endl;
105??
106??
107?????????????//Base2's?vtable
108??
109?????????????//pFun?=?(Fun)*((int*)*(int*)((int*)&d+1)+0);
110??
111?????????????pFun?=?(Fun)pVtab[1][0];
112??
113?????????????pFun();
114??
115??
116?????????????//pFun?=?(Fun)*((int*)*(int*)((int*)&d+1)+1);
117??
118?????????????pFun?=?(Fun)pVtab[1][1];
119??
120?????????????pFun();
121??
122?
123?????????????pFun?=?(Fun)pVtab[1][2];
124??
125?????????????pFun();?
126?
127??
128?????????????//The?tail?of?the?vtable
129??
130?????????????pFun?=?(Fun)pVtab[1][3];
131??
132?????????????cout<<pFun<<endl;
133??
134??
135?????????????//Base3's?vtable
136??
137?????????????//pFun?=?(Fun)*((int*)*(int*)((int*)&d+1)+0);
138??
139?????????????pFun?=?(Fun)pVtab[2][0];
140??
141?????????????pFun();
142??
143??
144?????????????//pFun?=?(Fun)*((int*)*(int*)((int*)&d+1)+1);
145??
146?????????????pFun?=?(Fun)pVtab[2][1];
147??
148?????????????pFun();
149??
150??
151?????????????pFun?=?(Fun)pVtab[2][2];
152??
153?????????????pFun();?
154?
155??
156?????????????//The?tail?of?the?vtable
157??
158?????????????pFun?=?(Fun)pVtab[2][3];
159??
160?????????????cout<<pFun<<endl;
161??
162??
163?????????????cout<<sizeof(d)<<endl;
164?
165?????????????return?0;
166??
167?}