菱形继承

例如：现在有五个类：人(Person)，学生(Student)，研究生(GStudent)，职工(Employee)，在职研究生(EGStudent)

#include
using namespace std;
#include//人
class Person 
{
private:int id;
public:Person(int pid) :id(pid) { cout << "create Person" << endl; }~Person() { cout << "destory Person" << endl; }
};//学生
class Student : public Person
{
private:int s_num;
public:Student(int pid, int sn) :Person(pid), s_num(sn) { cout << "create Student" << endl; }~Student() { cout << "destory Student" << endl; }
};//研究生
class GStudent : public Student
{
private:int g_num;
public:GStudent(int pid, int sn, int gn) :Student(pid, sn), g_num(gn) { cout << "create GStudent" << endl; }~GStudent() { cout << "destory GStudent" << endl; }
};//职工
class Employee : public Person
{
private:int e_num;
public:Employee(int pid, int en) :Person(pid), e_num(en) { cout << "create Employee" << endl; }~Employee() { cout << "destory Employee" << endl; }
};//在职研究生
class EGStudent : public Employee,public GStudent
{
private:int egs_num;
public:EGStudent(int spid,int epid, int en, int gn, int sn, int egn) :Employee(epid, en), GStudent(spid, sn, gn), egs_num(egn) {    cout << "create EGStudent" << endl;    }~EGStudent() { cout << "destory EGStudent" << endl; }
};int main()
{EGStudent egs(1, 2, 3, 4, 5, 6);return 0;
}

我们可以看到在职研究生对象egs共有3个成员属性

我们运行一下：

我们发现研究生对象egs是一个人，但却有两个身份证id，显然在语义层面是错误的。

这种问题我们一般称为数据冗余与二义性问题，为了解决这样的问题，我们引入了虚继承的概念。

虚继承

虚继承构造函数的调用顺序：

首先调用虚基类的构造函数（虚基类只会构建一次）

其次调用继承类的基类的构造函数

再调用继承类的构造函数

最后调用自己类的构造函数

我们将上述代码改为虚继承Person的方式：

#include
using namespace std;
#include//人
class Person 
{
private:int id;
public:Person(int pid) :id(pid) { cout << "create Person" << endl; }~Person() { cout << "destory Person" << endl; }
};//学生
class Student : virtual public Person
{
private:int s_num;
public:Student(int pid , int sn) :Person(pid), s_num(sn) { cout << "create Student" << endl; }~Student() { cout << "destory Student" << endl; }
};//研究生
class GStudent : public Student
{
private:int g_num;
public:GStudent(int pid, int sn, int gn) :Person(pid),Student(pid,sn), g_num(gn) { cout << "create GStudent" << endl; }~GStudent() { cout << "destory GStudent" << endl; }
};//职工
class Employee : virtual public Person
{
private:int e_num;
public:Employee(int pid, int en) :Person(pid), e_num(en) { cout << "create Employee" << endl; }~Employee() { cout << "destory Employee" << endl; }
};//研究生职工
class EGStudent : public Employee,public GStudent
{
private:int egs_num;
public:EGStudent(int epid,int spid,int en, int gn, int sn, int egn) :Person(epid),Employee(epid,en), GStudent(epid, sn, gn), egs_num(egn) {    cout << "create EGStudent" << endl;    }~EGStudent() { cout << "destory EGStudent" << endl; }
};int main()
{EGStudent egs(1, 2, 3, 4, 5, 6);return 0;
}