Dan Berliner

What is your goal? Are you simply looking to make a game or are you looking to know how to program? A graphical game will be well above the capabilities of a beginner programmer; there's just too much involved. If all you want to do is make a game use a tool like Game Maker. If you really want to do it with a programming language you are going to have the spend time getting better at general programming before doing something as ambitious as this. 

My code returns true only whan a's all fields are less than b's

This isn't a legal comparison for use with std::set. A valid comparison needs to be a strict weak ordering. One of the properties of a strict weak ordering is that if you have three objects A, B and C if A and B are equivalent and B and C are equivalent then A and C must also be equivalent. However, with this kind of comparison this property doesn't hold. One way to do this is to use a lexicographical comparison. Ex:

if (a.dir < b.dir) return true;
if (a.dir > b.dir) return false;
if (a.startx < b.startx) return true;
if (a.startx > b.startx) return false;
return a.starty < b.starty;

Interesting, thanks for sharing.

That's roughly the same idea, I like your approach better. Thanks.

The exact order it sorts in is unimportant, it just needs some way to do so.