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Bozebo

Unity [solved] pointer from stl list iterator?

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edit: this problem is solved, I am now asking for help here. Hi, firstly I will show an example application that represents my issue:
class cell{
  public:
    float top,left,bottom,right;

  //other methods and members

};

list<cell> cellList;
list<cell>::iterator cellIter;

//finds the cell that a point is in and returns a pointer to the cell
cell* findCell(float x,float y){
  for(cellIter = cellList.begin();cellIter != cellList.end();cellIter++){
    //if point is inside the cell
    if(x<=cellIter->right && x>=cellIter->left && y<=cellIter->top && y>=cellIter->bottom){
      break; //break the loop
    }
  }
  
  return (*cellIter);
}



I want findCell to return a pointer to the cell. Firstly I have to say that iterators confuse me a bit, they seem to be neither one thing nor the other, in the loop they work just like a pointer, but I can't return the iterator as the pointer, and I can't type cast it (tried a few other things untill I landed on the return line above). Also, cellList does not alter within the main loop, it is only edited on map initialisation. I really don't want to be passing object data around everywhere as that would be inefficient. (if it matters, I am programming this to learn how occlusion and collision grouping can be organised in an fps engine) What is the correct solution? Or should I find a different way to do the same task? edit: I found some discussions which spoke about the boost library, boost may shine light on any problems I have with lists of pointers but does it help me with this issue? [Edited by - Bozebo on April 25, 2010 8:30:44 PM]

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Quote:
Firstly I have to say that iterators confuse me a bit, they seem to be neither one thing nor the other,
in the loop they work just like a pointer, but I can't return the iterator as the pointer, and I can't type cast it (tried a few other things untill I landed on the code above).
As you note, although iterators can behave like pointers in some ways, they are not themselves pointers. (Well, strictly speaking, they might be in some cases, but as far as the user is concerned they have their own type, and aren't necessarily convertible to 'real' pointers.)

In this case though, you should be able to write:
return &(*cellIter);
Here, the 'deference' operator for the iterator is invoked, returning a reference to the pointed-at object. You can then take the address of the object as you would normally.

[Edit: I don't know that the Boost libraries have any direct relevance to the problem, as this is really just a basic issue of understanding how to use and manipulate iterators. I'll also mention that there are some other potential design issues here - such as the passing around of raw pointers, the brute-force search through the list of cells, and the use of list rather than, say, vector - that might warrant some examination once you get the basics working.]

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struct is_inside {
is_inside(float x_, float y_) : x(x_), y(y_) {}
bool operator()(cell & c) {
return (x >= c.left &&
x < c.right &&
y >= c.bottom &&
y < c.top);
}
private:
float x, y;
}

...
typedef std::list<cell> CellList;
CellList cellList;

CellList::iterator result = std::find_if(cellList.begin(), cellList.end(), is_inside(x,y));
if (result == cellList.end()) {
// not found
} else {
// result is first found cell
}




To find all cells that contain the point:
CellList::iterator result = cellList.begin();
while (result != cellList.end()) {
result = std::find_if(result, cellList.end(), is_inside(x,y));
if (result != cellList.end()) {
// (x,y) is inside result
}
}




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Quote:
Original post by Antheus
*** Source Snippet Removed ***

To find all cells that contain the point:
*** Source Snippet Removed ***
I'm not sure that those code samples address the OP's question. The OP was asking specifically about how to derive a pointer from a list iterator, which doesn't seem to be addressed at all in the above code samples.

Of course using algorithms from the standard library (as shown in your example code) is arguably preferable to writing out the loop manually, but I think that in most practical applications you'd want to use some sort of spatial hierarchy for this anyway (in other words, you probably wouldn't want to use a brute-force linear search for this, regardless of how the code for the search is written).

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Thanks for the help.

It is working nicely now, and I remembered that my cells are maintaining a list of adjacent cells (as is the map format). Rather than looping through all cells I should of definitely make it check only adjacent cells.

I thought about using vectors for that but I didn't see any reason to at the time. Is it that vectors are sequentially indexed (faster random access vs slower middle insertion) which makes them a better choice?

Should I use vectors or lists for my entities?
Noting that I need to order translucent drawing.

Should I maintain entities and their drawing system separately?

I love working on projects like this :D

(edit: oh now I have a segfault to deal with ^_^)

[Edited by - Bozebo on April 25, 2010 2:24:59 PM]

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