编程风格：object 带有引用或值的段落？ (c++)

Question

This is a general question on programming style. Let's say I have an object Line which has some methods and private variables Point point_a_ and Point point_b_ . Let's say that at some point I need to change the position of the two points. What programming style would you prefer between the following cases? They all do the same thing (or should do: I didn't compile, but seems pretty straightforward).

CASE 1

Class Line {
public:
  Line(Point point_a, Point point_b) : point_a_(point_a), point_b_(point_b) {}

  void UpdatePoints(Point point_a, Point point_b) { 
    point_a_ = point_a; point_b_ = point_b;
  }

  double Distance();

private:
  Point point_a_;
  Point point_b_;
};

int main (int argc, char * const argv[]) {
  Point point_a(0,0,0);
  Point point_b(1,1,1);
  Line line(point_a,point_b);
  std::cout<<line.Distance()<<"\n";

  point_a.x = 1;
  line.UpdatePoints(point_a,point_b);
  std::cout<<line.Distance()<<"\n";
}

CASE 2

Class Line {
public:
  Line(Point point_a, Point point_b) : point_a_(point_a), point_b_(point_b) {}
  Point& point_a() { return point_a_; }
  Point& point_b() { return point_b_; }
  double Distance();

private:
  Point point_a_;
  Point point_b_;
};

int main (int argc, char * const argv[]) {
  Point point_a(0,0,0);
  Point point_b(1,1,1);
  Line line(point_a,point_b);
  std::cout<<line.Distance()<<"\n";

  line.point_a().x = 1;
  std::cout<<line.Distance()<<"\n";
}

CASE 3

Class Line {
public:
  Line(Point* point_a, Point* point_b) : point_a_(point_a), point_b_(point_b) {}

  double Distance();

private:
  Point* point_a_;
  Point* point_b_;
};

int main (int argc, char * const argv[]) {
  Point point_a(0,0,0);
  Point point_b(1,1,1);
  Line line(&point_a,&point_b);
  std::cout<<line.Distance()<<"\n";

  point_a.x = 1;
  std::cout<<line.Distance()<<"\n";
}

Any feedback is greatly appreciated!!

Thanks!

[EDIT] Speed is paramount in my software!

Answer 1

In this simple scenario I might just use public member variables.

Otherwise I would provide getters that return a const reference and matching setters.

class Line {
public:
    Line(const Point& p1, const Point&p2) : m_p1(p1), m_p2(p2) {}

    const Point& p1() const 
    { return m_p1; }

    const Point& p2() const
    { return m_p2; }

    void setP1(const Point& p1)
    { m_p1 = p1; }

    void setP2(const Point& p2)
    { m_p2 = p2; }

private:
    Point m_p1;
    Point m_p2;
};

Answer 2

Case three is totally out because it completely violates principles of encapsulation. Case two does as well, to a slightly lesser extent. I would prefer option one, but did you consider possibly making the points immutable and forcing you to create a new object when it changes?

Also to be pedantic if I remember correctly from many years ago, a line technically extends infinitely in both directions. You're actually representing a line segment .

Answer 3

Case 2 is no better than public member variables. In particular, it doesn't encapsulate anything.

Case 3 makes ownership of the points unclear. Consider what happens if your points are local variables in the calling function, and then they go out of scope. It also offers no benefits over public member variables.

So out of these three options, Case 1 is the cleanest, IMO. Other options are:

• simply use public member variables.
• make Line immutable.
• use set and get functions.

Answer 4

I would choose either case 1 or an immutable Line class.

Case 2 allows changes to the Point objects without the knowledge of their containing line. At some point, you might need the line to know if its points have been changed.

Case 3 either makes the Line object dependent on the lifetime of the Points, or makes the Line the owner of the points, which is not clear from the API.

An immutable Line would allow you to create a new Line object with new points.

Answer 5

Case 4 -- Prefer passing by const reference rather than value (or pointer):

class Line
{
public:
    Line(const Point& a, const Point& b) : a_(a), b_(b)
    {}

    const Point& get_a() const { return a_; }
    const Point& get_b() const { return b_; }

    void set_a(const Point& a) { a_ = a; }
    void set_b(const Point& b) { b_ = b; }

private:
    Point a_;
    Point b_
};

This is better in that in enforces encapsulation - the only way to change the variables held in the class after construction is a specific mutator method.

The accessors return a const reference, so they cannot be modified (copies made from these can be).

The class as a whole is made const-correct.

References are arguably better than pointers in this instance because they are guaranteed (unless you specifically break that guarantee) to not be NULL.

Answer 6

Consider another option:

class Segment {
public:
    Segment(Point point_a, Point point_b);
    Point point_a() const;
    Point point_b() const;

private:
   Point point_a_;
   Point point_b_;
};


double Distance( Segment seg );

int main (int argc, char * const argv[]) {
    Point point_a(0, 0, 0);
    Point point_b(1, 1 ,1);
    Segment seg(point_a, point_b);
    std::cout << Distance(seg) << "\n";

    point_a.x = 1;
    seg = Segment(point_a, point_b); // reset
    std::cout << Distance(seg) << "\n";
}

I used the name Segment as per suggestion above. This style is closer to functional programming style. Segment is immutable unless you explicitly reset it with a common assignment syntax. Distance is not a member function because it can be implemented in terms of Segment's public interface.

Regards, &rzej