破除循环引用

在类外定义 friend 可破除循环引用。

#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;
}