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POHow am I accidentally overwriting when referencing these pointers?
 

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  1. POHow am I accidentally overwriting when referencing these pointers?
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    <p>Last question for tonight, I promise. These pointers are giving me a serious headache.</p> <p>I have a std::list<code>&lt;Point</code>> called Polygon and a std::list of Polygons defined like:</p> <pre><code>typedef std::list&lt;Point&gt; Polygon; typedef std::list&lt;Polygon&gt; PolygonList; // List of all our polygons PolygonList polygonList; </code></pre> <p>I created the method below to attempt to delete the nearest Point from an (x,y), checking all of my Polygons within my polygonList.</p> <pre><code>void deleteNearestPoint(int x, int y) { y = screenHeight - y; Polygon &amp;closestPolygon = polygonList.front(); Polygon::iterator closestPoint = closestPolygon.begin(); float closestDistance = sqrt(pow(x - closestPoint-&gt;x, 2) + pow(y - closestPoint-&gt;y, 2)); // Search PolygonList PolygonList::iterator listIter; Polygon::iterator iter; for(listIter = polygonList.begin(); listIter != polygonList.end(); listIter++) { Polygon &amp;tempPolygon = *listIter; for(iter = tempPolygon.begin(); iter != tempPolygon.end(); iter++) { const float distance = sqrt(pow(x - iter-&gt;x, 2) + pow(y - iter-&gt;y, 2)); if (distance &lt; closestDistance) { closestPolygon = *listIter; closestPoint = iter; closestDistance = distance; } } } closestPolygon.erase(closestPoint); redraw(); } </code></pre> <p>However, somewhere I have a pointer or reference variable screwing me up. This code compiles but acts in a very strange way.</p> <p>I've written a debug statement and lets say I have 3 polygons in my polygon list like so:</p> <blockquote> <p>Polygon #: 0<br> Point: (448, 43)<br> Point: (469, 177)<br> Point: (374, 123)<br> Polygon #: 1<br> Point: (295, 360)<br> Point: (422, 350)<br> Point: (315, 266)<br> Point: (295, 360)<br> Polygon #: 2<br> Point: (143, 202)<br> Point: (301, 203)<br> Point: (222, 100)<br> Point: (143, 202) </p> </blockquote> <p>Now, lets say I try and use the delete function giving it an x/y close to point 422, 350 The desired result would be it simply deleting that point (422, 350) from Polygon #1 but instead I get this:</p> <blockquote> <p>Polygon #: 0<br> Point: (295, 360)<br> Point: (422, 350)<br> Point: (315, 266)<br> Point: (295, 360)<br> Polygon #: 1<br> Point: (295, 360)<br> Point: (315, 266)<br> Point: (295, 360)<br> Polygon #: 2<br> Point: (143, 202)<br> Point: (301, 203)<br> Point: (222, 100)<br> Point: (143, 202) </p> </blockquote> <p>It did delete (422, 350) but it also has the strange side effect of overwriting Polygon#0 to what Polygon#1 was before the delete of its point.</p> <p>I know I'm using a pointer or reference incorrectly in my method. Can someone point out what I could possibly be doing that is causing this? I think it is because my &amp;closestPolygon is declared as a reference, but I get compile errors if I try to set it as anything else.</p>
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    1. USRex M
    1. USKingNestor
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    1. COAs an aside, and hopefully with some educational value: working with the squared distance will save you expensive sqrt() calls. Also, pow() is quite expensive; since you're only using integers, you're better off writing your own square() function.
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      1. POHow am I accidentally overwriting when referencing these pointers?
      1. USThomas
 

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