Derive Smiley from Circle

Question

I am trying to derive class Smiley from Circle:

struct Smiley : Circle {
  void draw_lines() const;
};

void Smiley::draw_lines() const {
  Circle::draw_lines();   // outline
  /*rest of code here*/
}

This is the definition of Circle:

struct Circle : Shape {
  Circle(Point p, int rr);    // center and radius

  void draw_lines() const;

  Point center() const;

  void set_radius(int rr) { set_point(0, Point(center().x - rr, center().y - rr)); r = rr; }
  int radius() const { return r; }
private:
  int r;
};

I basically want a Circle with a couple arcs drawn on top. Theoretically, I should only have to write draw_lines() over (which is defined as virtual in Shape), but it does nothing without a constructor, and if I have a constructor, it get an error and says that Circle does not have a default constructor available, even if I have no code in the constructor relating to Circle.

Does anyone know why I am getting this error?

But Circle does have a constructor, so why am I getting this error? I would just derive Smiley directly from Shape and redo the implementation details, but the assignment (not for class, just learning on my own) says to derive it from Circle. — , Dec 18 '13 at 18:36
@user2509848: It has a constructor, yes, and that constructor requires arguments. You're not giving it any. — Lightness Races in Orbit, Dec 18 '13 at 18:39

score 2 · Accepted Answer · answered Dec 18 '13 at 18:37

2

When you inherit from a class with no default constructor, you have to make sure you invoke the non-default one.

Otherwise your derived class tries to invoke that non-existent default constructor, as the error says. In this case, you can probably just pass the same arguments straight through.

struct Smiley : Circle {
  Smiley(Point p, int rr);
  void draw_lines() const;
};

Smiley::Smiley(Point p, int rr)
   : Circle(p, rr)
{};

I basically want a Circle with a couple arcs drawn on top

Then it's not a circle any more, so why does Smiley inherit Circle?

answered Dec 18 '13 at 18:37

Lightness Races in Orbit

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Because the outline of the face is a Circle. As I said in a comment above, this is an assignment. – Dec 18 '13 at 18:38
@user2509848: So the smiley _contains_ a circle, but it's not true that it _is_ a circle. I'd suggest composition rather than inheritance. – Lightness Races in Orbit Dec 18 '13 at 18:39
I do not know composition yet. The assignment is to inherit Circle (I am learning inheritance), but I will look into this. – Dec 18 '13 at 18:39
@user2509848: Yeah, fine, I'm just making sure you understand that. To my mind it's probably one of the most important lessons to learn from this assignment. :) – Lightness Races in Orbit Dec 18 '13 at 18:40
@user2509848: You are welcome. Did you understand _how_ it works? – Lightness Races in Orbit Dec 18 '13 at 18:41
Yes, Smiley's constructor calls Circle's constructor with the arguments to create it. I knew this needed to happen, but I didn't know how to make it happen. – Dec 18 '13 at 18:42
@user2509848: Okay. :) – Lightness Races in Orbit Dec 18 '13 at 18:43

score 1 · Answer 2 · edited May 23 '17 at 10:33

1

You've defined a constructor for circle that requires a Point and a Radius. You should define one with identical parameters for Smiley, that invokes the base class constructor with those parameters, via a Member Initialiser List.

struct Smiley : Circle {
  Smiley(Point p, int rr) : Circle(p, rr)
  {
  } 
  void draw_lines() const;
};

edited May 23 '17 at 10:33

Community

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answered Dec 18 '13 at 18:35

Roddy

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This can be better worded, esp. considering you're trying to explain to a novice. "You need to define an identical one for Smiley" is just plain wrong. – Luchian Grigore Dec 18 '13 at 18:36
So I just need to define a blank constructor for these problems? – Dec 18 '13 at 18:37
1

@user2509848 - the constructor isn't 'blank'. While it has an empty code body, the important thing is that is initializes the *base class* via an initialization list. – Roddy Dec 18 '13 at 18:41
Let us call it the _member initialiser list_; "initialization list" is now an overloaded term. – Lightness Races in Orbit Dec 18 '13 at 18:42
@LightnessRacesinOrbit. Agreed. Extra verbiage is always a good idea. (sighs deeply...) – Roddy Dec 18 '13 at 18:43
I see. Smiley's constructor invokes Circle's constructor with the arguments. – Dec 18 '13 at 18:43
@Roddy: Teehee ;) To be fair, the standard uses this terminology too, along with `mem-initializer-list` and `ctor-initializer` in the grammar. – Lightness Races in Orbit Dec 18 '13 at 18:43
@LuchianGrigore better now? ;-) – Roddy Dec 18 '13 at 18:45

Derive Smiley from Circle

2 Answers2