Question

Suppose I am defining a surface object

class surface
{
private:
    vector<point> points_;
    vector<hexFace> hexFaces_;
}

I have already written a point class, which is very necessary, but as for hexFace, actually it is very simple, it is just a list of four point labels, that is int[4]. And I don’t need to do any complex operation on it.

So my question is: what is the most efficient way in defining such an hexFace object. Should I use struct, or I’d better go with a class or anything else? What would you do? Thanks

And if I need to go with class, can I defining another class in the current class in a nesting way? If I can, do I have to write its constructors within this file also?

Does a struct need a constructor to initialize it?

Answer 1

You ask several questions:

what is the most efficient way in defining such an hexFace object.

Run-time efficiency will be about the same for any solution you choose. Lines-of-code efficiency, or maintainer-programmer-brain-power efficiency is probably more valuable.

If you are limited to pre-C++11 features, I’d use:

struct hexFace {
  int labels_[4];
};

If you can use C++11 features, try:

class surface
{
private:
    std::vector<point> points_;
    std::vector<std::array<int, 4>> hexFaces_;
}

Should I use struct, or I’d better go with a class or anything else?

struct and class are nearly synonymous. Use whichever you think expresses your intent more clearly. As for “something else”, try std::array

can I defining another class in the current class in a nesting way?

Yes, you can. Try:

class surface
{
private:
    class hexFace { public: int lables[4]; };
    vector<point> points_;
    vector<hexFace> hexFaces_;
};

If I can, do I have to write its constructors within this file also?

You may, or you may choose to write it elsewhere, or you may choose to omit the user-defined constructor altogether.

Here is how to write it inline:

class surface {
public:
    class hexFace { public: hexFace() { std::cout << "inline constructor!\n" } };
}

Here is how to write it externally

class surface {
  public:
  class hexFace {
    public:
      hexFace();
  };

// in another file ...
surface::hexFace::hexFace() { std::cout << "extern constructor\n"; }

Does a struct need a constructor to initialize it?

Neither a class nor a struct require a user-defined constructor, but both allow them.

struct X {
  X() { std::cout << "in struct constructor!\n"; }
};
class Y {
  public: 
    Y() { std::cout << "in class constructor!\n"; }
};