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I am currently in the process of coding a roguelike, and teaching myself what I need as I go.

I am stuck on a problem which I found somewhat answered here: Best way to organize entities in a game?

I need to have an entity tree, with classes as described in the best response to that question.

It mentions iterating through a std::set to run each entity's "script" - but leaves some things I need to know unexplained.

  1. In an implementation like that, how do I look something up with coordinates (ints in the class)? I suspect it's easier than running a For that iterates through everything until it finds the matching result...

  2. How would I handle multiple things being in the same spot (item stacks etc)? I know I can have multiple instances of a class contain the same coords, but what would I get back from a look-up in that kind of situation?

  3. Is there a way to iterate in, for example, descending order of an entity's "speed" stat?

Thanks~

Community
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motioneffector
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2 Answers2

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The best way to do it as you said, is to have a "global" array of entities (although please encapsulate them in a handler class or you will probably want to kill yourself eventually :p)

Have a base type entity, derive various things from that like crate and enemy and stuff, and maybe different classes derived from those for specialising different types (like grue and bat might have seperate classes? This can turn into a lot of different classes though so the best thing to do is usually have parameters per object (like for enemy they would have health, damage, speed etc)).

Store them all in the same array and have a virtual update function that you call on all of them every frame,

deek0146
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  • Sorry I think I misunderstood what you were thinking. When you said array of entities did you mean a 2d array representing a tileset? I meant just a vector of entity pointers, and the entity class has a position variable. I assume then that you are rendering this in ASCII characters? – deek0146 Mar 15 '11 at 07:51
  • It's an SDL tile engine game. I understand the derived classes thing, the problem I'm having is specifically with my numbered questions. Lookup by variables within the class. Such as "player is trying to move to 24,56 - is there an entity in 24,56?" "what items are at 24,55?" etc. when everything's held together by a std::set – motioneffector Mar 15 '11 at 07:55
  • Ah well if its tilebased thats different. Have a global array of entities like I said, but _also_ store them in a 2D array representing the tileset. And your problem with that was having more than one entity per tile? Have an std::vector of them for every tile and search through it checking for collision types. – deek0146 Mar 15 '11 at 08:13
  • is there no better way to check x,y for an entity than to run a loop that goes through every single entity until it finds one with those coordinates? – motioneffector Mar 15 '11 at 08:27
  • Read what I said. I said have a global array with every entity in it, and also have an array per tile containing all of the entities on that tile, and then check the list for that tile – deek0146 Mar 15 '11 at 08:49
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You'll be manipulating the entities through pointers, so there's no problem putting a single entity in several containers. So for 1) Matrix > level; (where Matrix is a simple two dimensional array you implement yourself), This also handles more than one element at a given place. And level[row][column] would return a reference to an std::vector. (Actually, I'd probably wrap the std::vector in a custom class, say EntityList, with a very simple interface. This keeps my options open for the future.)

With regards to 3, the simplest solution is simply to copy the list into a local EntityList, and sort that according to whatever criteria you want.

James Kanze
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  • Yes but it would most likely be useful to also have all of the entities in a global array as well, otherwise to update them he would have to iterate through every tile in the matrix searching for entities. – deek0146 Mar 15 '11 at 09:33
  • @Alasdair There are probably a couple of other containers as well; I just mentionned the one he asked about. My point was that you can put a pointer to the entity into multiple containers without a problem. – James Kanze Mar 15 '11 at 18:19