向量的指针问题

Question

我在尝试将自定义类的对象 Push_back 到以我的自定义类作为类型的指针向量时遇到了相当大的麻烦。 请参阅下面的代码以及收到的错误。 我在 Windows XP 上使用带有 CDT 插件的 Eclipse 和 OpenCV。

我花了很多时间试图找到答案但毫无结果！ PS我是一名学生，指针等不是我的事！

    std:: vector<RoadLine>* LaneChangeDetector::roadLines(IplImage* img_8uc1, IplImage* img_8uc3, IplImage* img_edge, std::vector <RoadLine>* roadVector){

    CvMemStorage* storage = cvCreateMemStorage(0);
    CvSeq* lines = 0;
    CvMemStorage* roadStorage = cvCreateMemStorage(0);
    CvSeq* roadLines = 0;

    // Probabalistic Hough transform returns line segments from edge detected image
    lines = cvHoughLines2( img_edge, storage, CV_HOUGH_PROBABILISTIC, 1, CV_PI/180, 50, 200, 200 );

    // Sequence roadlines, lines with correct slope are added to this sequence
    roadLines = cvCreateSeq(0, lines->header_size, lines->elem_size, roadStorage);

    // slope
    double m = 0.0;

    // Point of intersection
    CvPoint poi;

    for(int i = 0; i < lines->total; i++ ){
        CvPoint* line = (CvPoint*)cvGetSeqElem(lines,i);
        CvPoint pt1 = line[0];
        CvPoint pt2 = line[1];

        double x1 = double(pt1.x);
        double y1 = double(pt1.y);
        double x2 = double(pt2.x);
        double y2 = double(pt2.y);

        if(pt1.x == pt2.x){
            m = 1.0;
        }
        else{
            m = (double(y2 - y1)/(double(x2 - x1)));
        }

        if( ((m>0.45) && (m<0.75)) || ((m<-0.45) && (m>-0.75)) ){

            // If the slope is between measured parameters add to roadLines sequence for further analysis
            cvSeqPush(roadLines, line);
        }
    }

    // otherRoadLine used for comparison
    CvPoint* otherRoadLine;

    for(int a=0; a<roadLines->total; a++){

        CvPoint* roadLine = (CvPoint*)cvGetSeqElem(roadLines,a);
        CvPoint rl1 = roadLine[0];
        CvPoint rl2 = roadLine[1];
        int lineCount = 0;

        if(a>0){

            // Test the current line against all the previous lines in the sequence.
            // If the current line is far enough away from all other lines then draw it
            for(int b=0; b<a; b++){
                otherRoadLine = (CvPoint*)cvGetSeqElem(roadLines,b);
                if((roadLine->x > ((otherRoadLine->x) + 200)) || (roadLine->x < ((otherRoadLine->x) - 200)) ){
                    lineCount++;
                }
            }
            if(lineCount == a){
                cvLine(img_final, roadLine[0], roadLine[1], CV_RGB(0,0,255), 3, CV_AA, 0 );
                RoadLine myLine = RoadLine(roadLine, 1);
                roadVector->push_back(myLine); //ERROR OCCURS HERE
                cvShowImage("Plate Detection", img_final);
                cvWaitKey(0);
            }
        }
        else{
            cvLine(img_final, roadLine[0], roadLine[1], CV_RGB(0,0,255), 3, CV_AA, 0 );
            RoadLine myLine = RoadLine(roadLine, 1);
            roadVector->push_back(myLine //ERROR OCCURS HERE
            cvShowImage("Plate Detection", img_final);
            cvWaitKey(0);
        }
    }

    if(roadVector->size() >= 2){
        int pos = 0;
        RoadLine line1 = roadVector->at(pos);
        RoadLine line2 = roadVector->at(pos + 1);

        CvPoint* A = line1.line;
        CvPoint p1 = A[0];
        CvPoint p2 = A[1];

        int A1 = p1.y - p2.y;
        int B1 = p1.x - p2.x;
        int C1 = (p1.x*p2.y) - (p1.y*p2.x);

        CvPoint* B = line2.line;
        CvPoint p3 = B[0];
        CvPoint p4 = B[1];

        int A2 = p3.y - p4.y;
        int B2 = p3.x - p4.x;
        int C2 = (p3.x*p4.y) - (p3.y*p4.x);

        int det = A2*B1 - A1*B2;

        if(det == 0){
            printf("Lines are parallel");
        }
        else{
            int x = ( C1*(p3.x - p4.x) - (p1.x - p2.x)*C2 )/det;
            int y = ( C1*(p3.y - p4.y) - (p1.y - p2.y)*C2 )/det;

            poi.x = x;
            poi.y = y;

            horizon = poi.x;

            cvCircle(img_final, poi, 10, CV_RGB(255, 0, 0), 2, CV_AA, 0);
        }
    }

    cvShowImage("Plate Detection", img_final);
    cvWaitKey(0);

    return roadVector;
}

可以在此处看到自定义类 RoadLine

    #include <cv.h>
class RoadLine{
private:
CvPoint* line;
int lane;
public:
RoadLine(CvPoint*, int);
};
RoadLine::RoadLine(CvPoint* aLine, int aLane){
line = aLine;
lane = aLane;
}

从调试中我可以看到“std::vector* roadVector”正在正确初始化。

以下是 Eclipse 告诉我的内容：

3 std::vector<RoadLine, std::allocator<RoadLine> >::push_back() F:\MinGW\include\c++\3.4.5\bits\stl_vector.h:560 0x0043e3f9

4 void std::_Construct<RoadLine, RoadLine>() F:\MinGW\include\c++\3.4.5\bits\stl_construct.h:81 0x0044015d  

程序跳转到 stl_construct.h 中的这部分代码

  template<typename _T1, typename _T2>
inline void
_Construct(_T1* __p, const _T2& __value)
{
  // _GLIBCXX_RESOLVE_LIB_DEFECTS
  // 402. wrong new expression in [some_]allocator::construct
  ::new(static_cast<void*>(__p)) _T1(__value); //DEBUG THROWS ME TO THIS LINE
}

再次，任何帮助将不胜感激。

干杯

帕特

Answer 1

您不使用指针向量，而是使用对象向量。 在这种情况下，您的类需要有一个复制构造函数，因为 Push_back 存储对象的副本。

作为一般的调试建议，尝试通过删除尽可能多的代码来解决问题，但仍然会看到不正确的行为。 尝试找到失败的最简单的例子。

Answer 2

你的新 RoadLine 类肯定会导致灾难：

RoadLine::RoadLine(CvPoint* aLine, int aLane){
    line = aLine;
    lane = aLane;
}

RoadLine::RoadLine(const RoadLine & myRoadLine){
    line = myRoadLine.line;
    lane = 1;
}

RoadLine::~RoadLine(){
    delete line;
}

使用它的代码：

                        if(lineCount == a){
                                cvLine(img_final, roadLine[0], roadLine[1], CV_RGB(0,0,255), 3, CV_AA, 0 );
                                RoadLine myLine = RoadLine(roadLine, 1);//create object on the Stack
                                roadVector->push_back(myLine); //Push COPY of myLine
                                cvShowImage("Plate Detection", img_final);
                                cvWaitKey(0);
                        }//Stack-based object "myLine" is automatically destroyed here (leaves scope)

“myLine”的自动销毁将删除“myLine.line”（在 RoadLine 的 dtor 中）
但向量中仍然引用“myLine.line”（您刚刚推送了它）。

您必须对行进行深度复制（正如其他人建议的那样），如下所示：

RoadLine::RoadLine(const RoadLine & myRoadLine){
    line = new CvPoint(*myRoadLine.line);//assuming CvPoint can be copy-constructed
    lane = 1;
}

或者使用 CvLine 对象而不是指针（或其他东西，需要更多上下文）

编辑：
Dirk Gently 的 copy-ctor 有一个 bug，因为它会将内存泄漏给前一个“line”成员
应该是：

RoadLine& operator=(const RoadLine & o){
     if (this != &o) { //Remember to check for self-assignment.
      delete []line;//delete[] vs. delete !
      line = 0;//if next line throws at least we won't double-delete line
      line = new CvPoint[ 2 ]; //this might throw ! should catch (or redesign to get rid of new (prefered)
      line[ 0 ] = o.line[ 0 ];
      line[ 1 ] = o.line[ 1 ];
      lane = o.lane;
     }
     return *this;
}
//consistent constructor !
RoadLine::RoadLine(CvPoint* aLine, int aLane)
    :line(new CvPoint[2]),//might throw, but its better to throw in initializer ! (if you just have one pointer it might be ok to do it like this)
    lane(aLane)
{
     line[0] = aLine[0];
     line[1] = aLine[1];
}
RoadLine::~RoadLine(){
    delete[] line;//also use delete[] vs. normal delete here !
}

编辑2：我几乎忘记了我知道为什么它会崩溃！ 也许你尝试用最后一个和最后一个+1 CvPoint 构建一对（就像这个明显错误的代码）？

CvPoint Pnts[2] = {CvPoint(0,0),CvPoint(1,1)};
Roadline Line(&Pnts[1],1);//tries to access Pnts[2] which is one past end !

Answer 3

您的 RoadLine 类缺少适当的复制因子。 现在，由于您有一个指向 CvPoint 对象的成员，因此您每次 push_back 时都会创建指针的副本。 这可能是不可取的。

RoadLine::RoadLine(const RoadLine & o){
     line = new CvPoint[ 2 ]; 
     line[ 0 ] = o.line[ 0 ];
     line[ 1 ] = o.line[ 1 ];
     lane = o.lane;
}

RoadLine& operator=(const RoadLine & o){
     if (this != &o) { //Remember to check for self-assignment.
      line = new CvPoint[ 2 ]; 
      line[ 0 ] = o.line[ 0 ];
      line[ 1 ] = o.line[ 1 ];
      lane = o.lane;
     }
     return *this;
}

缩短代码：尝试隔离问题：

int main() {
    CvPoint pa[] = { CvPoint(0, 0), CvPoint(100, 100) };
    RoadLine rl1(pa, 1);

    vector<RoadLine> v;
    v.push_back(rl1);

    return 0;
}

这会崩溃吗？

Answer 4

C++ 的技巧是把“~”键想象成又大又红，只要你按下它，闹钟就会响起，即。 每当您考虑向类添加析构函数时。

如果您要添加析构函数，那么您需要一个复制构造函数和赋值运算符。 没有例外。 即使您不打算复制该对象，您仍然应该在私有部分中声明它们，这样如果意外使用它们，编译器就会给出错误。

当对象的生命周期受到控制时，您还应该使用引用计数指针而不是原始 C 风格指针（在 C++ 中，这是“RAII”）。 如果你这样做，析构函数将从 RoadLine 中消失，神奇的是，你的问题也会消失。

Answer 5

你没有指针向量。

std::vector<RoadLine>* roadVector

是指向 RoadLine 对象向量的指针。 如果您想要一个指针向量，您应该这样做：

std::vector<RoadLine*> roadVector

这可能会对您有所帮助（因为向量将不再调用复制构造函数），但您仍然应该按照其他人的建议对它们进行排序。

Answer 6

此类错误通常是由不正确的内存管理引起的。 遗憾的是，您还没有发布如何管理记忆的方法。

如果你可以让它在 Linux 系统上运行，你可以尝试在 valgrind 下运行你的程序，这有助于追踪不正确的内存访问/释放。 不幸的是，valgrind 在 windows 下不可用，但可能有替代品。

Answer 7

我已将 RoadLine 的类定义更改为：

#include <cv.h>

class RoadLine{

private:
    int lane;
public:
    CvPoint* line;
    RoadLine(CvPoint*, int);
    RoadLine(const RoadLine &);
    ~RoadLine();
    RoadLine& operator=(const RoadLine & o);
};

RoadLine::RoadLine(CvPoint* aLine, int aLane){
    line = aLine;
    lane = aLane;
}

RoadLine::RoadLine(const RoadLine & myRoadLine){
    line = new CvPoint[ 2 ]; // CRASHES HERE
    line[ 0 ] = myRoadLine.line[ 0 ];
    line[ 1 ] = myRoadLine.line[ 1 ];
    //line = new CvPoint(*myRoadLine.line);
    lane = myRoadLine.lane;
}

RoadLine::~RoadLine(){
    delete line;
}

RoadLine& RoadLine::operator=(const RoadLine & o){
     if (this != &o) { //Remember to check for self-assignment.
      line = new CvPoint[ 2 ];
      line[ 0 ] = o.line[ 0 ];
      line[ 1 ] = o.line[ 1 ];
      lane = o.lane;
     }
     return *this;
}

这是 RoadLine 类的当前版本

这就是我实现该类的方式：

else{
        cvLine(img_final, roadLine[0], roadLine[1], CV_RGB(0,0,255), 3, CV_AA, 0 );
        RoadLine myLine(roadLine, 1);
        roadVector->push_back(myLine); // FROM HERE
        cvShowImage("Plate Detection", img_final);
        cvWaitKey(0);
}

当调用 push_back 时，它会调用复制构造函数，但程序在上面突出显示的地方崩溃了

有什么区别事实上我的向量已定义；

std::vector<RoadLine>* roadVector

我有一个 CvPoint* 而不是 CvPoint[]

抱歉，如果这些看起来非常基本的问题