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C++中是否允許一個類繼承多個父類？

C++支持編寫多重繼承的代碼：

• 一個子類可以擁有多個父類
• 子類擁有所有父類的成員變量
• 子類繼承父類所有的成員函數
• 子類對象可以當做任意父類對象使用

多重繼承的語法規則：

多重繼承的本質與單繼承相同

編程示例;

#include <iostream> #include <string>using namespace std;class BaseA {int ma; public:BaseA(int a){ma = a;}int getA(){return ma;} };class BaseB {int mb; public:BaseB(int b){mb = b;}int getB(){return mb;} };class Derived : public BaseA, public BaseB {int mc; public:Derived(int a, int b, int c) : BaseA(a), BaseB(b){mc = c;}int getC(){return mc;}void print(){cout << "ma = " << getA() << ", "<< "mb = " << getB() << ", "<< "mc = " << mc << endl;} };int main() {cout << "sizeof(Derived) = " << sizeof(Derived) << endl; // 12Derived d(1, 2, 3);d.print();cout << "d.getA() = " << d.getA() << endl;cout << "d.getB() = " << d.getB() << endl;cout << "d.getC() = " << d.getC() << endl;cout << endl;BaseA* pa = &d;BaseB* pb = &d;cout << "pa->getA() = " << pa->getA() << endl;cout << "pb->getB() = " << pb->getB() << endl;cout << endl;void* paa = pa;void* pbb = pb;if( paa == pbb ){cout << "Pointer to the same object!" << endl; }else{cout << "Error" << endl;}cout << "pa = " << pa << endl;cout << "pb = " << pb << endl;cout << "paa = " << paa << endl;cout << "pbb = " << pbb << endl; return 0; }

運行結果：

分析以上程序我們就可以發現問題所在啦：
1、通過多重繼承的對象可能擁有不同的地址

2、多重繼承可能產生冗余的成員：

當多重繼承關系閉合將產生數據冗余問題
解決辦法是：
虛繼承！！！

下面看一個解決冗余的例子：

#include <iostream> #include <string>using namespace std;class People {string m_name;int m_age; public:People(string name, int age){m_name = name;m_age = age;}void print(){cout << "Name = " << m_name << ", "<< "Age = " << m_age << endl;} };class Teacher : virtual public People { public:Teacher(string name, int age) : People(name, age){} };class Student : virtual public People { public:Student(string name, int age) : People(name, age){} };class Doctor : public Teacher, public Student { public:Doctor(string name, int age) : Teacher(name, age), Student(name, age), People(name, age){} };int main() {Doctor d("Delphi", 33);d.print();return 0; }

運行結果為;
Name = Delphi, Age = 33

虛繼承能夠解決數據冗余的問題

中間層父類不再關心頂層父類的初始化

最終子類必須直接調用頂層父類的構造函數

雖然我們解決的數據冗余，但是還有一個問題，在架構設計師，無法確定使用虛繼承還是直接繼承？？？

3、多重繼承有可能會產生多個虛函數表

#include <iostream> #include <string>using namespace std;class BaseA { public:virtual void funcA(){cout << "BaseA::funcA()" << endl;} };class BaseB { public:virtual void funcB(){cout << "BaseB::funcB()" << endl;} };class Derived : public BaseA, public BaseB {};int main() {Derived d;BaseA* pa = &d;BaseB* pb = &d;BaseB* pbe = (BaseB*)pa; // oops!!BaseB* pbc = dynamic_cast<BaseB*>(pa); //dynamic_cast會對指針進行修正cout << "sizeof(d) = " << sizeof(d) << endl;cout << "Using pa to call funcA()..." << endl;pa->funcA();cout << "Using pb to call funcB()..." << endl;pb->funcB();cout << "Using pbc to call funcB()..." << endl;pbc->funcB();cout << endl;cout << "pa = " << pa << endl;cout << "pb = " << pb << endl;cout << "pbe = " << pbe << endl;cout << "pbc = " << pbc << endl;return 0; }

工程開發中的多繼承方式：

#include <iostream> #include <string>using namespace std;class Base { protected:int mi; public:Base(int i){mi = i;}int getI(){return mi;}bool equal(Base* obj){return (this == obj);} };class Interface1 { public:virtual void add(int i) = 0;virtual void minus(int i) = 0; };class Interface2 { public:virtual void multiply(int i) = 0;virtual void divide(int i) = 0; };class Derived : public Base, public Interface1, public Interface2 { public:Derived(int i) : Base(i){}void add(int i){mi += i;}void minus(int i){mi -= i;}void multiply(int i){mi *= i;}void divide(int i){if( i != 0 ){mi /= i;}} };int main() {Derived d(100);Derived* p = &d;Interface1* pInt1 = &d;Interface2* pInt2 = &d;cout << "p->getI() = " << p->getI() << endl; // 100pInt1->add(10);pInt2->divide(11);pInt1->minus(5);pInt2->multiply(8);cout << "p->getI() = " << p->getI() << endl; // 40cout << endl;cout << "pInt1 == p : " << p->equal(dynamic_cast<Base*>(pInt1)) << endl;cout << "pInt2 == p : " << p->equal(dynamic_cast<Base*>(pInt2)) << endl;return 0; }

運行結果：

p->getI() = 100
p->getI() = 40

pInt1 == p : 1
pInt2 == p : 1

一些有用的工程建議：

先繼承自一個父類，然后實現多個接口

父類中提供equal()成員函數

equal()成員函數用于判斷當前指針是否指向當前對象

與多重繼承相關的強制類型轉換用dynamic_cast完成

總結：

多繼承中可能出現多個虛函數表指針

與多重繼承相關的強制類型轉換用dynamic_cast完成

工程開發中使用單繼承多接口的方式實現多繼承

父類提供成員函數用來判斷指針是否指向當前對象

總結

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