1 C++中的多重继承
C++支持编写多重继承的代码：
- 一个子类可以拥有多个父类。
- 子类拥有所有父类的成员变量。
- 子类继承所有父类的成员函数。
- 子类对象可以当作任意父类对象使用。
多重继承的语法规则：
2 C++中多重继承带来的问题
2.1 同一个对象的地址不同
看如下代码：
#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; // 12
Derived 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;
}
执行结果如下：
问题分析：
2.2 带来数据冗余的问题
多重继承可能带来冗余的成员：
实现代码如下：
#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 : public People
{
public:
Teacher(string name, int age) : People(name, age)
{
}
};
class Student : 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 + "1", age), Student(name + "2", age)
{
}
};
int main()
{
Doctor d("Delphi", 33);
//d.print(); // 编译error，因为成员函数冗余，编译器不知道调用哪个
d.Teacher::print();
d.Student::print();
return 0;
}
打印结果：
当多重继承关系出现闭合时将产生数据冗余的问题！
解决方案：虚继承
对于虚继承：
- 虚继承能够解决数据冗余的问题。
- 中间层父类不再关心顶层父类的初始化。
- 最终子类必须直接调用顶层父类的构造函数。
解决方案代码：
#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;
}
在架构设计中需要继承时，我们是无法确定使用直接继承还是虚继承！虚继承会使得架构设计可能出现问题。
2.3 可能产生多个虚函数表
多重继承可能产生多个虚函数表：
示例代码如下：
#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* pbb = (BaseB*)pa; // oops!!
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;
pbb->funcB();
cout << endl;
return 0;
}
输出结果如下：
修改后的代码：
#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);
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;
}
输出结果：
3 工程开发中多重继承的方式
工程中我们使用如下“单继承多接口”的方式使用多重继承：
对于正确的使用多重继承，一些有用的工程建议：
- 先继承自一个父类，然后实现多个接口。
- 父类中提供equal()成员函数。
- equal()成员函数用于判断指针是否指向当前对象。
- 与多重继承相关的强制类型转换用dynamic_cast完成。
示例代码：
#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; // 100
pInt1->add(10);
pInt2->divide(11);
pInt1->minus(5);
pInt2->multiply(8);
cout << "p->getI() = " << p->getI() << endl; // 40
cout << endl;
cout << "pInt1 == p : " << p->equal(dynamic_cast<Base*>(pInt1)) << endl;
cout << "pInt2 == p : " << p->equal(dynamic_cast<Base*>(pInt2)) << endl;
return 0;
}
参考资料：
