friend

由于循环引用，下面的程序出现了问题。

#include <iostream>

using namespace std;

class Sniper;

class Supplier
{
    int storage;

public:
    Supplier(int storage = 1000) : storage(storage) {}

    // bool foo(Sniper & sniper){sniper.bullets++;}
    bool provide(Sniper &sniper);
};

class Sniper
{
private:
    int bullets;

public:
    Sniper(int bullets = 0) : bullets(bullets) {}
    // friend class Supplier;
    friend bool Supplier::provide(Sniper &sniper);
};

bool Supplier::provide(Sniper &sniper)
{
    // bullets is a private member
    if (sniper.bullets < 20) // no enough bullets
    {
        if (this->storage > 100)
        {
            sniper.bullets += 100;
            this->storage -= 100;
        }
        else if (this->storage > 0)
        {
            sniper.bullets += this->storage;
            this->storage = 0;
        }
        else
            return false;
    }
    cout << "sniper has " << sniper.bullets << " bullets now." << endl;
    return true;
}

int main()
{
    Sniper sniper(2);
    Supplier supplier(2000);
    supplier.provide(sniper);
    return 0;
}