Inheritance question

[Hypnotik]

So I'm working on a program. Base class is a polygon (which has an unknown number of sides), with derived classes for rectangle and triangle which inherit from the polygon, and square which inherits from triangle. In my base class I have 2 private members: num_sides and length. Also in my base class I have a setnum_sides() and setlength() function. Do I simply call these 2 functions to set the num_sides and length for all shapes? It seems a little redundant to enter 4 lengths for rectangle, since there are 2 sides of each length.

This is a handwritten program, but if someone needs the code I have so far posted I can type it up....just might take a little bit.

Thanks,
J

[ilikepython]

So I'm working on a program. Base class is a polygon (which has an unknown number of sides), with derived classes for rectangle and triangle which inherit from the polygon, and square which inherits from triangle. In my base class I have 2 private members: num_sides and length. Also in my base class I have a setnum_sides() and setlength() function. Do I simply call these 2 functions to set the num_sides and length for all shapes? It seems a little redundant to enter 4 lengths for rectangle, since there are 2 sides of each length.

This is a handwritten program, but if someone needs the code I have so far posted I can type it up....just might take a little bit.

Thanks,
J

Shouldn't square derive from rectangle instead of triangle? What is lenght supposed to do? Maybe it's better to have a int * pointer lenghts to store the lenght of all the sides. For the sides, you can override the functions in derived classes that you use to set them. For example, the square setSides would take 1 integer compared to the rectangle's 2.

[Hypnotik]

I would assume that square would derive from rectangle, but it's not what the assignment says. I'm assuming that is a mistake. The program has length involved because the next assignment is going to calculate area and perimeter of the shapes.

[epyk]

I would assume that square would derive from rectangle, but it's not what the assignment says. I'm assuming that is a mistake. The program has length involved because the next assignment is going to calculate area and perimeter of the shapes.

well in actuality square and rectangle can inherit from triangle. this will work if triangle contains virtual function for area and perimeter has length, width and hyp. square can inherit them and have its own VIRTUAL function for determining its area and parimeter... esentially a polygon is made up of differant triangles, which an instance of a triangle can contain 1 or more other triangles... look at any 3-d game or model skeleton, you will not see a square.

[JosAH]

Shouldn't square derive from rectangle instead of triangle?

A square should not even derive from a rectangle. Read my LSP article in the
Java Article section about exactly this little issue.

kind regards,

Jos

[ilikepython]

A square should not even derive from a rectangle. Read my LSP article in the
Java Article section about exactly this little issue.

kind regards,

Jos

Nice article, just thought I'd post the link.

[Hypnotik]

How would this work? I ask in main the number of sides. If 4 is selected I ask if it is a polygon, a rectangle or a square, then call the respective setlength function. The setlength function would be declared in the polygon class, the the 3 derived classes would inherit just that. If 3 is selected it would call the triangle.setlength, and if a number greater than 4 is selected it would call the polygon.setlength function? I'm just confused as to how I would inherit the numsides function from polygon, if I don't know the number of sides.

J

[JosAH]

How would this work? I ask in main the number of sides. If 4 is selected I ask if it is a polygon, a rectangle or a square, then call the respective setlength function. The setlength function would be declared in the polygon class, the the 3 derived classes would inherit just that. If 3 is selected it would call the triangle.setlength, and if a number greater than 4 is selected it would call the polygon.setlength function? I'm just confused as to how I would inherit the numsides function from polygon, if I don't know the number of sides.

J

Trying to determine the actual shape given just the number of sides is not a
good selection mechanism. You need a better user input processing mechanism
than just that.

kind regards,

Jos

[weaknessforcats]

Trying to determine the actual shape given just the number of sides is not a
good selection mechanism.

That should be workable. You have a base class Shape and derived classes ThreeSides, FourSides, etc.

Rectangle derives from FourSides
Pararellogram derived from FourSides
etc.

Then you just:

Shape* s = new Rectangle;
 
s->DisplayName();     //you see "Rectangle"
s->DisplayNumberSides();   //you see 4
 

That is, you do not try to determine the shape by the number of sides. The object already knows how many sides it has so you just ask it for that information.