Re: C++ify tag

red floyd <>
Fri, 06 Jun 2014 09:46:23 -0700
On 6/6/2014 3:03 AM, wrote:

I've got a collection of lines, which intersect with each other. To avoid numerical problems and other nasty things, I want to store the intersections as intersections, rather than x,y coordinates.

Now each line means something to higher-level code. It could be an index in a polygon, part of a space invader, whatever. So it needs a arbitrary tag to retrieve and query it.

Now in C I'd write this something like the following.

struct line
   double x1, y1, x2, y2;
   void *tag;
   int taglen;
   int hash;
   struct line **intersections;
   int N_intersections;

Then you store the LINE objects in a hash table to retrieve them.

The interface is then

addline(x1, y1, x2, y2, tag, taglen);
LINE *getintersections(tag, taglen);

But the program contains C++, it would be nice to have a more C++ interface to it, so that the tags can contain embedded strings or whatever. Also maybe some sort of template so an int tag and a string tag which happens to be 4 bytes and the same bit pattern as the int doesn't evaluate to the same thing.

why not make line a base class, remove the tag/taglen fields,
and use inheritance and virtual methods to 'do the right thing'
with the line.


struct line {
virtual line* getintersections() = 0;

struct polygon_index : public line {
line *getintersections(); // do polygon index stuff

struct space_invader: public line {
line *getintersections(); // do space invader stuff

And avoid the LINE stuff, as a matter of style, ALLCAPS
should pretty much only be used for #defines.

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