Thread: Constructor Exercise difficulty.

Hi guys,

Having a little difficulty with an exercise from a book that I am working through. Currently looking at classes using a very simple graphics library: circles, polygons, lines, etc.

An example they have used is a Polygon class. You simply create as Polygon poly. And then call a member function to add points to the object to create the shape.

The exercise is to create a similar class that passes the points to the constructor to ensure that there are more than 2 points, ie, to ensure it fits with geometric rules about what a polygon is. My question is how do I check an arbitrary number of points within a constructor?

Obviously, I know how to pass a set number of arguments, it would simply look as such:

Code:

poly::poly(Point a, Point b, Point c)
{
//code
}

Any help is appreciated.

Thanks

Can you not pass a container of points into the constructor. Unless I am missing the point, I would add a number of points to e.g. a STL vector, format

std::vector<point> myVec;

(assuming 'point' is a defined class). Then you can iterate over the vector and do whatever you need to. Obvisouly, if you just wanted to check the number of points passed, you can call the size() method on the vector.

Originally Posted by Elysia

Are you saying your constructors needs to accept a variable amount of points, with a requirement that it be > 2?

Yes, that's what I am thinking.

Originally Posted by CodeMonkey62

Can you not pass a container of points into the constructor. Unless I am missing the point, I would add a number of points to e.g. a STL vector, format

std::vector<point> myVec;

(assuming 'point' is a defined class). Then you can iterate over the vector and do whatever you need to. Obvisouly, if you just wanted to check the number of points passed, you can call the size() method on the vector.

I did think of vectors, would this require a vector to be created outside the scope of the class and then passed by ref to the constructor?

Originally Posted by Elysia

Yes, it would. And that would be the best solution available today.
In coming versions, we should get initializer lists which should do exactly what you require. But we're not there yet.

The reason why I didn't implement this, is that I thought this would have broken the encapsulation rule?

As a newcomer to programming, I thought classes/objects etc, was to ensure all data/functions are self contained.

Is it okay to break that convention?

Originally Posted by darren78

I did think of vectors, would this require a vector to be created outside the scope of the class and then passed by ref to the constructor?

Yes, and sorry, I should have mentioned the reference passing, we don't want any unecessary copying, now, do we.

Also Elysia, I notice that you are using C++0x. Is there much difference in that that the previous standard? I'm a little worried that some of the stuff that I am learning may be becoming redundant?

Thanks

Originally Posted by darren78

The reason why I didn't implement this, is that I thought this would have broken the encapsulation rule?

As a newcomer to programming, I thought classes/objects etc, was to ensure all data/functions are self contained.

Is it okay to break that convention?

Well, your points are all being created outside the poly class, all you are doing is collecting them together to pass in a single, variable sized container. If you want your class to have copies of the data for itself, as private members with appropriate accessors, then you'll need to implement this in the poly class.

Okay, I understand now. Thanks for the help.