cppPointDistanceFriendFunction

#include <iostream>

class ClassB
{
    void bFunction() {/* тело функции */};
};

class ClassName
{
    // определение класса

    friend void nonMemberFunction(); // 1
    friend void ClassB::bFunction(); // 2
};

void nonMemberFunction() {/* тело функции */ }

class Point
{
    int x;
    int y;
public:
    Point() = default;
    Point(int pX, int pY) : x{ pX }, y{ pY } {}
    Point& setX(int pX) { x = pX; return *this; }
    Point& setY(int pY) { y = pY; return *this; }
    int getX()const { return x; }
    int getY()const { return y; }
    void showPoint() const
    {
        std::cout << '(' << x << ',' << y << ')';
    }
    friend double distance(const Point& p1, const Point& p2);
};

double distance(const Point& p1, const Point& p2)
{
    auto xLength{ p2.x - p1.x };
    auto yLength{ p2.y - p1.y };
    return sqrt(xLength * xLength + yLength * yLength);
}

int main()
{
    Point p1{ 5,5 };
    Point p2{ 10,10 };

    std::cout << "Distance between p1 and p2 is: " << distance(p1, p2) << '\n';

    return 0;
}