C++ 友谊
示例
该friend关键字是用来给其他类和函数访问类的private和protected成员，甚至通过他们的class`s范围之外定义。
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
//允许在这里访问私有成员。
std::cout <<animal.weight<< ", " <<animal.height<< std::endl;
}
}
std::ostream& operator<<(std::ostream& os, Animal animal)
{
os << "动物身高： " <<animal.height<< "\n";
return os;
}
int main() {
Animal animal = {10, 5};
printWeight(animal);
AnimalPrinter aPrinter;
aPrinter.print(animal);
std::cout << animal;
}10 10, 5 动物身高： 5