友誼

friend 關鍵字用於為其他類和函式提供對類的私有成員和受保護成員的訪問許可權，即使它們是在類的範圍之外定義的。

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