友谊

friend 关键字用于为其他类和函数提供对类的私有成员和受保护成员的访问权限，即使它们是在类的范围之外定义的。

class Animal{
private:
    double weight;
    double height;
public:
    friend void printWeight(Animal animal);
    friend class AnimalPrinter;
    // A common use for a friend function is to overload the operator<< for streaming. 
    friend std::ostream& operator<<(std::ostream& os, Animal animal);
};

void printWeight(Animal animal)
{
    std::cout << animal.weight << "\n";
}

class AnimalPrinter
{
public:
    void print(const Animal& animal)
    {
        // Because of the `friend class AnimalPrinter;" declaration, we are
        // allowed to access private members here.
        std::cout << animal.weight << ", " << animal.height << std::endl;
    }
}

std::ostream& operator<<(std::ostream& os, Animal animal)
{
    os << "Animal height: " << animal.height << "\n";
    return os;
}

int main() {
    Animal animal = {10, 5};
    printWeight(animal);

    AnimalPrinter aPrinter;
    aPrinter.print(animal);

    std::cout << animal;
}

10
10, 5
Animal height: 5