關於“野指標”的問題

最近在用Qt做一個程式，其中就涉及到了“野指標”的，果然出了問題（為什麼要用果然呢？ 因為以前看過林銳博士的一篇文章提到過這個問題），多次調試檢查後才解決。現記錄如下：

問題描述：程式的目的大概是一個讀檔案資料並在QGraphicsScene上畫出圖形再刪掉這些圖形。程式每運行一次，雖然調用QGraphicsScene的clear()清除items，但是記憶體佔用增加大概300K左右。

問題分析：程式中有一個迴圈（大概4000次左右，根據檔案大小不同而不同），在迴圈中我new了一個對象，在迴圈結束時delete了這個對象。 但是並沒有把對象指標賦為NULL，導致每迴圈一遍產生了一個野指標，從而記憶體不斷增加。以下是程式片段：

while(!in.atEnd())
                        {
                           QPainterPath *pLinePath = new QPainterPath();
                           QVector <QPointF> *plinePoint = new QVector <QPointF>;

                           in>>drawItem;
                           inName>>itemName;

                           L5RoadName.append(itemName);
                           if(drawItem=="LINE" || drawItem=="Line")
                                {
                                   plineNode=2;
                                }
                            else if(drawItem=="PLINE" || drawItem=="Pline")
                                {
                                    in>>plineNode;
                                    in.flush();

                                }
                            else
                                break;

                            for(int i=0; i!=plineNode; i++)
                                {
                                    in>>P1_x>>P1_y;
                                    in.flush();
                                    testPoint.setX(P1_x * 100000000.0);
                                    testPoint.setY(20000000000.0-P1_y * 100000000.0);
                                    plinePoint->append(testPoint);
                                
                                }
                            for(int i=0; i<(plineNode); i++)
                                {
                                    if(0==i)
                                        {
                                            testpoint1=plinePoint->first();
                                            pLinePath->moveTo(testpoint1.x(),testpoint1.y());
                                        }
                                    else
                                        {
                                            testpoint1=plinePoint->at(i);
                                            pLinePath->lineTo(testpoint1.x(),testpoint1.y());
                                        }
                                }

                            L5RoadPath.append( *pLinePath);
                            in>>symbol;
                            delete plinePoint;
                            delete pLinePath;
                            plinePoint=NULL; 
                            pLinePath=NULL; 
                        }

 在增加了最後兩條語句後再調試，發現記憶體佔用不再增長。