c++之派生类访问基类的友元函数

#include <iostream>
#include <string>

using std::cout;
using std::cin;
using std::endl;
using std::string;

class BaseClass
{
private:
string m_name;
public:
BaseClass(string name);
BaseClass(){}
virtual ~BaseClass(){}
friend std::ostream & operator << (std::ostream & os , const BaseClass & b);
};
class DerivedClass : public BaseClass
{
private:
int m_age;
public:
DerivedClass(int age, string name);
DerivedClass(){}
virtual ~DerivedClass(){}
friend std::ostream & operator << (std::ostream & os , const DerivedClass & d);
};

BaseClass::BaseClass(string name) : m_name(name)
{
}

std::ostream & operator << (std::ostream & os,const  BaseClass & b)
{
os << "m_name: " << b.m_name << endl;
return os;
}

DerivedClass::DerivedClass(int age, string name):
BaseClass(name)
{
m_age = age;
}

std::ostream & operator << (std::ostream & os , const DerivedClass & d)
{
os << (const BaseClass & ) d;//在这里进行了显式的类型转化
os << "m_age: " << d.m_age << endl;
}

int main()
{
DerivedClass  d(22,"郭小郭");
cout << d << endl;
return 0;
}