friend2.cpp

#include <iostream>
using namespace std;

class Supplier;
class Sniper;

class Supplier
{
    int storage;
public:
    Supplier(int storage = 1000): storage(storage){}
    bool provide(Sniper & sniper);
};

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