使用友元函数和运算符重载

友元函数

类的友元函数可以访问其任何成员。也就是说，尽管Peek()函数不是Critter类的成员函
数，但它还是可以访问Critter的任意成员——Peek()利用这种关系访问私有数据成员
m_Name。

friend void Peek(const Critter& aCritter);
...

void Peek(const Critter& aCritter)
{
    cout << aCritter.m_Name << endl;
}

当 main 函数调用 Peek() 时，crit的私有数据成员 m_Name 被显示出来。

Peek(crit)

运算符重载

重载运算符的方法：定义一个名为 operatorX 的函数，其中 X 是需要重载的运算符。

ostream& operator<<(ostream& os, const Critter& aCritter)
{
    os << "Critter Object - ";
    os << "m_Name: " << aCritter.m_Name;
    return os;
}

该函数之所以能访问 Critter 对象的私有数据成员 m_Name，是因为下面这行代码
使得该函数成为 Critter 类的友元:

friend ostream& operator<<(ostream& os, const Critter& aCritter);

源码

#include <iostream>
#include <string>
using namespace std;
class Critter
{
    friend void Peek(const Critter& aCritter);
    friend ostream& operator<<(ostream& os, const Critter& aCritter);
public:
    Critter(const string& name = "");
private:
    string m_Name;
};

Critter::Critter(const string& name) :
    m_Name(name)
{}

void Peek(const Critter& aCritter);
ostream& operator<<(ostream& os, const Critter& aCritter);

int main()
{
    Critter crit("Poochie");
    cout << "Calling Peek() to access crit's private data member, m_Name: \n";
    Peek(crit);
    cout << "\nSending crit object to cout with the << operator:\n";
    cout << crit;

    return 0;
}

// global friend function that can access all of a Critter object's members
void Peek(const Critter& aCritter)
{
    cout << aCritter.m_Name << endl;
}

// global friend function that can access all of a Critter object's members
ostream& operator<<(ostream& os, const Critter& aCritter)
{
    os << "Critter Object - ";
    os << "m_Name: " << aCritter.m_Name;
    return os;
}