在一次修改代碼過程中踩的坑，下來研究了一下，發(fā)現(xiàn)C++中虛函數(shù)重載后會(huì)產(chǎn)生很多有意思的情況，在這里總結(jié)了一下。

C++中有重載（overload）和重寫（override）以及重定義這幾個(gè)概念，1 overload：指的是相同作用域中的兩個(gè)函數(shù)的函數(shù)名相同，但參數(shù)列表的個(gè)數(shù)、順序、類型不同。而override指的是子類中重新定義的父類的虛函數(shù)。2 override：overload要求兩個(gè)函數(shù)的參數(shù)列表必須不同，但是不要求這兩個(gè)函數(shù)必須是虛函數(shù)。而override要求必須是虛函數(shù)且父類的虛函數(shù)必須有virtual關(guān)鍵字，函數(shù)的參數(shù)列表和返回值也必須相同。子類中override的虛函數(shù)的訪問修飾符可以不同。3 重定義也是描述分別位于父類與子類中的同名函數(shù)的，但返回值可以不同。如果參數(shù)列表不同，這時(shí)子類中重定義的函數(shù)不論是否有virtual關(guān)鍵字，都會(huì)隱藏父類的同名函數(shù)。如果參數(shù)列表相同，但父類中的同名函數(shù)沒有virtual關(guān)鍵字修飾，此時(shí)父類中的函數(shù)仍然被隱藏。

虛函數(shù)的典型用法是：

#include <iostream>using namespace std; class Base{public:    virtual void f();};void Base::f(){    cout << "Base::f()" << endl;}class Derived:public Base{public:    virtual void f();};void Derived::f(){    cout << "Derived::f()" << endl;}int main() {    Base *p1 = new Base;    Base *p2 = new Derived;    p1->f();    p2->f();        delete p1;    delete p2;    return 0;}
程序的輸出是：
Base::f()Derived::f()
那么如果虛函數(shù)之間又發(fā)生了overload，會(huì)出現(xiàn)什么情況？我們先看最簡單的情況：
#include <iostream>using namespace std; class Base{public:    virtual void f(int);};void Base::f(int a){    cout << "Base::f(int) " << a << endl;}class Derived:public Base{public:    virtual void f();};void Derived::f(){    cout << "Derived::f()" << endl;}int main() {    Base *p1 = new Base;    p1->f(1);        Base *p2 = new Derived;    p2->f();        delete p1;    delete p2;        return 0;}
編譯上面的代碼，會(huì)發(fā)生如下錯(cuò)誤：
test.cpp: In function ‘int main()’:test.cpp:28:11: error: no matching function for call to ‘Base::f()’     p2->f();           ^test.cpp:10:6: note: candidate: virtual void Base::f(int) void Base::f(int a){      ^test.cpp:10:6: note:   candidate expects 1 argument, 0 PRovided
這就是因?yàn)楦割愔刑摵瘮?shù)的參數(shù)列表已經(jīng)發(fā)生變化，這時(shí)不論子類中重定義的函數(shù)不論是否有virtual關(guān)鍵字，都會(huì)隱藏父類的同名函數(shù)。這時(shí)子類中只是重定義了一個(gè)自己的函數(shù)virtual void f()，而并沒有override父類中對(duì)應(yīng)的虛函數(shù)。p2是一個(gè)指向Base類型的指針，根據(jù)虛函數(shù)的特性，對(duì)p2→f();的處理取決于是否override了父類的虛函數(shù)，如果沒有，仍然會(huì)調(diào)用調(diào)用父類中被override的虛函數(shù)，但是現(xiàn)在父類中的函數(shù)已經(jīng)成為了virtual void f(int)，因此在執(zhí)行p2→f()時(shí)會(huì)由于缺少輸入?yún)?shù)而出現(xiàn)上述錯(cuò)誤。為了證明上述論斷，可以在執(zhí)行p2→f()時(shí)傳入?yún)?shù)來判斷：
#include <iostream>using namespace std; class Base{public:    virtual void f(int);};void Base::f(int a){    cout << "Base::f(int) " << a << endl;}class Derived:public Base{public:    virtual void f();};void Derived::f(){    cout << "Derived::f()" << endl;}int main() {    Base *p1 = new Base;    p1->f(1);        Base *p2 = new Derived;    p2->f(2);        delete p1;    delete p2;        return 0;}
這時(shí)可以通過編譯，執(zhí)行結(jié)果為
Base::f(int) 1Base::f(int) 2
可以看到，子類自己定義的virtual void f()其實(shí)是父類的virtual void f(int)的一個(gè)重定義的函數(shù)，這時(shí)盡管p2實(shí)際指向了一個(gè)Derived對(duì)象，但由于沒有override父類對(duì)應(yīng)的虛函數(shù)，在執(zhí)行 p2→f(2)時(shí)將執(zhí)行父類的virtual void f(int)。也可以這樣修改：
#include <iostream>using namespace std; class Base{public:    virtual void f(int);    virtual void f();};void Base::f(int a){    cout << "Base::f(int) " << a << endl;}void Base::f(){    cout << "Base::f() " << endl;}class Derived:public Base{public:    virtual void f();};void Derived::f(){    cout << "Derived::f()" << endl;}int main() {    Base *p1 = new Base;    p1->f(1);        Base *p2 = new Derived;    p2->f();        delete p1;    delete p2;        return 0;}
因?yàn)楦割愔卸x了可被子類override的函數(shù)，所以這時(shí)執(zhí)行p2→f()又會(huì)重新執(zhí)行子類的virtual void f()：
Base::f(int) 1Derived::f()
我們甚至還可以這樣驗(yàn)證：
#include <iostream>using namespace std; class Base {public:    virtual void f(int);    virtual void f();};void Base::f(int a) {    cout << "Base::f(int) " << a << endl;}void Base::f() {    cout << "Base::f() " << endl;}class Derived:public Base {public:    virtual void f(float);};void Derived::f(float a) {    cout << "Derived::f(float)" << showpoint << a << endl;}int main() {    Base *p1 = new Base;    p1->f(1);        Base *p2 = new Derived;    p2->f(**2.0**);        delete p1;    delete p2;        return 0;}
這時(shí)輸出仍然為
Base::f(int) 1Base::f(int) 2
這說明如果通過指向父類的指針，調(diào)用虛函數(shù)時(shí)，如果子類重定義了該虛函數(shù)（參數(shù)列表發(fā)生變化），則實(shí)際調(diào)用的仍是父類中的虛函數(shù)。上面都是通過指向父類的指針來調(diào)用虛函數(shù)的，那么如果通過指向子類的指針調(diào)用虛函數(shù)會(huì)發(fā)生什么：
#include <iostream>using namespace std; class Base {public:    virtual void f(int);    virtual void f();};void Base::f(int a) {    cout << "Base::f(int) " << a << endl;}void Base::f() {    cout << "Base::f() " << endl;}class Derived:public Base {public:    virtual void f(float);};void Derived::f(float a) {    cout << "Derived::f(float)" << showpoint << a << endl;}int main() {    Base *p1 = new Base;    p1->f(1);        Derived *p2 = new Derived;    p2->f(2.0);    p2->f(3);        delete p1;    delete p2;        return 0;}
這時(shí)輸出就變?yōu)榱耍?/p>
Base::f(int) 1Derived::f(float)2.00000Derived::f(float)3.00000
這說明，如果通過指向子類的指針調(diào)用虛函數(shù)，并且子類重定義了父類的虛函數(shù)，這時(shí)實(shí)際調(diào)用的就將是子類中的虛函數(shù)。