一、 C++成員函數(shù)的重載

C++中的成員函數(shù)有四種，分別是普通成員函數(shù)，virtual虛函數(shù)，const成員函數(shù)。

(1) void func(int a);(2) virtual void func(int a);(3) void func(int a) const;

如果在一個(gè)類中，聲明這四種函數(shù)，哪是重復(fù)定義？哪些是重載？

其中(1)(2)是重復(fù)定義，故編譯不能通過，而(3)與(1)(2)是不同類型的函數(shù)，是重載。

成員函數(shù)被重載的特征是：

(1)具有相同的作用域(即同一個(gè)類定義中)；(2)函數(shù)名字相同(3)參數(shù)類型，順序 或 數(shù)目不同(包括const參數(shù)和非const函數(shù))(4)virtual關(guān)鍵字可有可無。

從成員函數(shù)的重載特征中，可以知道(1)(2)是重復(fù)定義。那么(3)為什么和(1)(2)不同呢？

因?yàn)轭愔械暮瘮?shù)，都會自動(dòng)添加一個(gè)自身類指針this，所以

void func(int a) ==== void func(Base * this, int a)virtual func(int a) ==== virtual func(Base *this, int a)void func(int a)const === void func(const Base *this, int a) const

所以(3)可以與(1)(2)發(fā)生重載，因?yàn)閰?shù)有一個(gè)const。

二 、C++成員函數(shù)的繼承#include <iostream> using namespace std; class Base { public:     void f(int a){         cout << "Base::f(int a)" << endl;     }     virtual void g(int a) {         cout << "virtual Base::g(int a)" << endl;     } }; class Derived : public Base { public:     void h(int a) {         cout << "Derivd::h(int a)" << endl;     } }; int main() {     Base b;     b.f(3);     b.g(4);     Derived d;     d.f(3);     d.g(4);     d.h(3); } #include <iostream>using namespace std;class Base {public: void f(int a){  cout << "Base::f(int a)" << endl; } virtual void g(int a) {  cout << "virtual Base::g(int a)" << endl; }};class Derived : public Base{public: void h(int a) {  cout << "Derivd::h(int a)" << endl; }};int main(){ Base b; b.f(3); b.g(4); Derived d; d.f(3); d.g(4); d.h(3);}

Base b的對象模型：

Derived d的對象模型：

則在子類Derived d中繼承了父類中的virtual void g(int a) ; void f(int a);

運(yùn)行結(jié)果為：

三、C++成員函數(shù)的覆蓋

覆蓋是指派生類重新實(shí)現(xiàn)(或者改寫)了基類的成員函數(shù)，其特征是：

(1)不同的作用域(非別位于派生類和基類中)；(2)函數(shù)名稱相同(3)參數(shù)列表完全相同；(4)基類函數(shù)必須是虛函數(shù)。

從(4)中我們得知覆蓋只是針對虛函數(shù)的。

#include <iostream> using namespace std; class Base { public:     void f(int a){         cout << "Base::f(int a)" << endl;     }     virtual void g(int a) {         cout << "virtual Base::g(int a)" << endl;     } }; class Derived : public Base { public:     void h(int a) {         cout << "Derivd::h(int a)" << endl;     }     virtual void g(int a) {         cout << "virtual Derived::g(int a)" << endl;     } }; int main() {     Base b;     b.f(3);     b.g(4);     Derived d;     d.f(3);     d.g(4);     d.h(3); } #include <iostream>using namespace std;class Base {public: void f(int a){  cout << "Base::f(int a)" << endl; } virtual void g(int a) {  cout << "virtual Base::g(int a)" << endl; }};class Derived : public Base{public: void h(int a) {  cout << "Derivd::h(int a)" << endl; } virtual void g(int a) {  cout << "virtual Derived::g(int a)" << endl; }};int main(){ Base b; b.f(3); b.g(4); Derived d; d.f(3); d.g(4); d.h(3);}

Derived d對象模型如下：

其中Derived中重新定義了基類的虛成員函數(shù)virtual void g(int a)；

四、C++成員函數(shù)的隱藏

隱藏是指派生類的成員函數(shù)遮蔽了與其同名的基類成員函數(shù)，具體規(guī)則如下：

(1) 派生類的函數(shù)與基類的函數(shù)同名，但是參數(shù)列表有所差異。此時(shí)，不論有無virtual關(guān)鍵字，基類的函數(shù)在派生類中將被隱藏。(注意別與重載混合)

(2)派生類的函數(shù)與基類的函數(shù)同名，參數(shù)列表也相同，但是基類函數(shù)沒有virtual關(guān)鍵字。此時(shí)，基類的函數(shù)在派生類中將被吟唱。(注意別與覆蓋混合)

判斷下面哪些函數(shù)是覆蓋，哪些函數(shù)是隱藏？

#include <iostream> using namespace std; class Base { public:     virtual void f(float x) {         cout << "virtual Base::f(float) " << x << endl;     }     void g(float x) {         cout << "Base::g(float) " << x << endl;     }     void h(float x) {         cout << "Base::h(float) " << x << endl;     } }; class Derived : public Base{ public:     virtual void f(float x) {         cout << "virtual Derived::f(float) " << x << endl;     }     void g(int x) {         cout << "Derived::g(int) " << x << endl;     }     void h(float x) {         cout << "Derived::h(float) " << x << endl;     } }; int main(void) {     Derived d;     Base *pb = &d;     Derived *pd = &d;     pb->f(3.14f);     pd->f(3.14f);     pb->g(3.14f);     pd->g(3.14f);     pb->h(3.14f);     pd->h(3.14f); } #include <iostream>using namespace std;class Base {public: virtual void f(float x) {  cout << "virtual Base::f(float) " << x << endl; } void g(float x) {  cout << "Base::g(float) " << x << endl; } void h(float x) {  cout << "Base::h(float) " << x << endl; }};class Derived : public Base{public: virtual void f(float x) {  cout << "virtual Derived::f(float) " << x << endl; } void g(int x) {  cout << "Derived::g(int) " << x << endl; } void h(float x) {  cout << "Derived::h(float) " << x << endl; }};int main(void){ Derived d; Base *pb = &d; Derived *pd = &d; pb->f(3.14f); pd->f(3.14f); pb->g(3.14f); pd->g(3.14f); pb->h(3.14f); pd->h(3.14f);}

其中子類Derived中 vitual void f(float x)  是覆蓋，而void g(int x) 和void h(float x)都是隱藏。

運(yùn)行結(jié)果：

再看一個(gè)例子：

#include <iostream> using namespace std; class Base { public:     virtual void f(int a) {         cout << "virtual Base::f(int a)" << endl;     }     void f(double d) {         cout << "Base::f(double d)" << endl;      } }; class Derived : public Base { public:     void f(double d) {         cout << "Derivd::f(double d)" << endl;     } }; int main() {     Derived d;     d.f(3);     d.f(2.5);     Derived *pd = new Derived();     pd->f(3);     pd->f(2.5);     Base b;     b.f(5);     b.f(3.5);     Base *pBase = new Derived();     pBase->f(5);     pBase->f(3.5); } #include <iostream>using namespace std;class Base{public: virtual void f(int a) {  cout << "virtual Base::f(int a)" << endl; } void f(double d) {  cout << "Base::f(double d)" << endl; }};class Derived : public Base{public: void f(double d) {  cout << "Derivd::f(double d)" << endl; }};int main(){ Derived d; d.f(3); d.f(2.5); Derived *pd = new Derived(); pd->f(3); pd->f(2.5); Base b; b.f(5); b.f(3.5); Base *pBase = new Derived(); pBase->f(5); pBase->f(3.5);}

其中父類中的void f(double d)隱藏了子類的virtual void f(int a),  void f(double d)函數(shù)。

所以在主函數(shù)中

Derived d; d.f(3); d.f(2.5); Derived *pd = new Derived(); pd->f(3); pd->f(2.5); Derived d;d.f(3);d.f(2.5);Derived *pd = new Derived();pd->f(3);pd->f(2.5);只要通過Derived對象或者Derived指針執(zhí)行f()函數(shù)，都只執(zhí)行void Derived::f(double d)該函數(shù)。[html]Base *pBase = new Derived(); pBase->f(5); pBase->f(3.5); Base *pBase = new Derived();pBase->f(5);pBase->f(3.5);

在調(diào)用pBase->f(5)時(shí)，首先要去pBase類中找到對應(yīng)需要執(zhí)行的函數(shù)，因?yàn)锽ase類中有兩個(gè)函數(shù)virtual void f(int a) 和 void f(double)重載，因?yàn)樵搶?shí)參是5，為int類型，所以要調(diào)用virtual void f(int a)函數(shù)，因?yàn)樵揻(int a)是一個(gè)虛函數(shù)，所以再去判斷pBase所指向的具體對象，具體對象為Derived子類，再去Derived子類的虛函數(shù)表中找到void f(int a)函數(shù)。因?yàn)镈erived子類繼承了父類Base的虛函數(shù)vitural void f(int a)，所以輸出 virtual Base::f(int a);

在調(diào)用pBase->f(3.5)時(shí)，首先要去pBase類中找到對應(yīng)需要執(zhí)行的函數(shù)，因?yàn)橐驗(yàn)锽ase類中有兩個(gè)函數(shù)virtual void f(int a) 和 void f(double)重載，因?yàn)樵搶?shí)參是3.5，為double類，所以要調(diào)用void f(double d)函數(shù)，因?yàn)樵摵瘮?shù)是一個(gè)普通成員函數(shù)，故直接輸出。 void Base::f(double d);

再舉一個(gè)例子：

#include <iostream> using namespace std; class Base { public:     virtual void f(int a) {         cout << "virtual Base::f(int a)" << endl;     }     void f(double d) {         cout << "Base::f(double d)" << endl;      } }; class Derived : public Base { public:     void f(int a) {         cout << "virtual Derived::f(int a)" << endl;     } }; int main() {     Derived d;     d.f(3);     d.f(2.5);     Derived *pd = new Derived();     pd->f(3);     pd->f(2.5);     Base b;     b.f(5);     b.f(3.5);     Base *pBase = new Derived();     pBase->f(5);     pBase->f(3.5); } #include <iostream>using namespace std;class Base{public: virtual void f(int a) {  cout << "virtual Base::f(int a)" << endl; } void f(double d) {  cout << "Base::f(double d)" << endl; }};class Derived : public Base{public: void f(int a) {  cout << "virtual Derived::f(int a)" << endl; }};int main(){ Derived d; d.f(3); d.f(2.5); Derived *pd = new Derived(); pd->f(3); pd->f(2.5); Base b; b.f(5); b.f(3.5); Base *pBase = new Derived(); pBase->f(5); pBase->f(3.5);}

子類Derived中的void f(int a)既覆蓋了基類Base的虛函數(shù)virtual void f(int a)，也隱藏了基類的virtual void f(int a),  void f(double d)函數(shù)。

Derived d; d.f(3); d.f(2.5); Derived *pd = new Derived(); pd->f(3); pd->f(2.5); Derived d;d.f(3);d.f(2.5);Derived *pd = new Derived();pd->f(3);pd->f(2.5);

同理所有用子類對象或者子類指針來調(diào)用f()函數(shù)時(shí)，都只執(zhí)行virtual void f(int a)，輸出virtual Derived::f(int a)

view plaincopyprint?Base *pBase = new Derived(); pBase->f(5); pBase->f(3.5); Base *pBase = new Derived();pBase->f(5);pBase->f(3.5);

pBase->f(5)，首先去Base類中尋找相應(yīng)的函數(shù)，同理Base類中的兩個(gè)函數(shù)virtual void f(int a)和void f(double d)是重載函數(shù)，因?yàn)閷?shí)參為5,為int類型，所以我們要調(diào)用virtual void f(int a)虛函數(shù)，因?yàn)樵摵瘮?shù)是虛函數(shù)，所以要去判斷pBase指向的具體對象，因?yàn)閜Base指向的是一個(gè)子類的對象，所以需要去子類的虛函數(shù)表中取找virtual void f(int a)函數(shù)，找到之后，執(zhí)行該函數(shù)，故輸出virtual Derived::f(int a)。

pBase->f(3.5),首先去Base類中尋找相應(yīng)的函數(shù)，同理Base類中的兩個(gè)函數(shù)virtual void f(int a)和void f(double d)是重載函數(shù)，因?yàn)閷?shí)參為3.5,為double類型，所以我們要調(diào)用void f(double d)，因?yàn)樵摵瘮?shù)為普通成員函數(shù)，故執(zhí)行輸出： void Base::f(double d);

作者：黑馬程序員C++培訓(xùn)學(xué)院    
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