无法成功创建唯一对象的std::列表

Question

我有一个基于OpenGL GLUT的C++程序（是的，我知道，GLUT很古老），它交替打印出一个蓝色矩形和一个红色正方形，位于GLUT窗口中用户单击鼠标的位置。这很好用。

然而，当我试图修改上面的程序，使上面提到的形状在初始动画后仍保留在画布上时，我遇到了令人难以置信的麻烦。

我创建了一个Shape类，它包含诸如顶点数量、颜色、坐标等信息。当程序一次只绘制一个形状时，该类似乎完全可用。

为了同时解决多个形状的问题，我创建了一个std::list<Shape>链表。然而，当我通过std::list<Shape>::iterator机制遍历链表时，对象似乎在内存中绑定在一起。也就是说，迭代为链表中的每个索引生成完全相同的形状对象、坐标等等。

我尝试了以下解决方案：

使链接列表为std::list<Shape*>而不是std::list<Shape>，

• 利用堆经由CCD_ 5进行对象分配
• 并将上述两种方法相结合

这是我的void display() GLUT函数，以及相关的全局变量和类定义/声明：

class Shape
{
public:
    Shape();
    Shape(int, double[], int[]);
    int type; //0 = rectangle, 1 = circle, 2 = triangle
    int numVertices; //stores total number of vertices in the 2D object.
    double* vertexArray; //a dynamic array that stores each vertex's (x, y)-coordinates in alternating successive indices
    int* rgb; //an array that contains the 3 rgb values s.t. rgb = {r, g, b}
    double* center; //an array that contains the (x, y)-coordinates of the shape's center on the 2d plane.
    int velocity[2]; //a array of size 2 that holds the object's x-velocity in index 0 and the object's y-velocity in index 1
};
    //default Shape constructor
Shape::Shape()
{
}
//Shape constructor 
Shape::Shape(int shapeType, double vertices[], int color[]) //constructor for creating a stationary 2D shape
{
    type = shapeType;
    if (shapeType!=1) //as long as shape is NOT a circle, interpret the second constructor parameter as a list of vertices 
    {
        vertexArray = vertices;
    }
    rgb = color;
    if (shapeType==0) //shape is a rectangle
    {
        numVertices = 4;
    }
    else if(shapeType==1) //shape is a circle
    {
        //shape is a circle, therefore the second array param is in fact an array of size 2 containing the (x, y)-coordinates of the circle origin...
        center = vertices;
    }
    else if (shapeType==2) //shape is a triangle
    {
        numVertices = 3; 
    }
}

std::list<Shape> shapeList;
void my_display(void) 
{
  /* clear the buffer */
  glClear(GL_COLOR_BUFFER_BIT);

  //altFlag is just a value to allow alternating between rectangles/circles being printed
  if (altFlag==1)
  {
      printf("Drawing rectangle at: (%g, %g)n", my_x, my_y);
      /*instantiate a Shape() object representing a blue rectangle*/
      int rgbColor[3] = {0, 0, 1};
      double vertices[4] = {my_x/window_w, my_y/window_h, my_x/window_w + my_rect_w, my_y/window_h + my_rect_h};
      Shape my_rectangle(0, vertices, rgbColor); //uses constructor (shape type, list of vertex coordinates, length of coordinate list, color)
      glColor3f((GLfloat)my_rectangle.rgb[0], (GLfloat)my_rectangle.rgb[1], (GLfloat)my_rectangle.rgb[2]) ; /* (Red, Green, Blue); so here we ask for Blue */
      glRectf(my_rectangle.vertexArray[0], my_rectangle.vertexArray[1], my_rectangle.vertexArray[2], my_rectangle.vertexArray[3]); //call to function to draw a rectangle
      altFlag=0;
      shapeList.push_front(my_rectangle);
  }
  else
  {
      /*instantiate a Shape() object representing a red circle*/
      int circleColor[3] = {1, 0, 0};
      double circleCenter[2] = {(my_x), (my_y)}; //{center x coord, center y coord}
      //Shape* my_circle = new Shape(1, circleCenter, circleColor); 
      Shape my_circle(1, circleCenter, circleColor);
      glColor3f(my_circle.rgb[0], my_circle.rgb[1], my_circle.rgb[2]);
      glCirclef(my_circle.center[0], my_circle.center[1]); //call to function to draw pseudocircle
      altFlag=1;
      shapeList.push_front(my_circle);
  }
  //iterate over shapeList, print out values of the rgb array.
  for (std::list<Shape>::iterator iter = shapeList.begin(); iter != shapeList.end(); iter++)
  {
      printf("%d, %d, %dn", iter->rgb[0], iter->rgb[1], iter->rgb[2]);
  }
  glutSwapBuffers();
  return;

虽然这是作业的一部分，但问题与所使用的语言有关，而不是作为课程重点的图形库。

Answer 1

Shape的参数化构造函数可以执行以下操作：

vertexArray = vertices;

其中vertices是声明为double vertices[]的函数参数。这是double *vertices的语法糖，即，您正在复制指针（因为数组在传递给函数时会衰减为指针）。

CCD_ 11被定义为类内部的指针。您甚至提到它是一个"动态数组"，但随后您使用堆栈上分配的数组（display()中的局部变量）对其进行初始化。这会使指针悬空。

您必须使vertexArray成为一个数组并复制数据，而不仅仅是指针。类似这样的东西：

class Shape
{
  // ...
  double vertexArray[4];
  // ...
};
Shape::Shape(int shapeType, double vertices[], int color[])
{
    type = shapeType;
    if (shapeType==0) //shape is a rectangle
    {
        numVertices = 4;
    }
    else if(shapeType==1) //shape is a circle
    {
        //shape is a circle, therefore the second array param is in fact an array of size 2 containing the (x, y)-coordinates of the circle origin...
        center = vertices;
    }
    else if (shapeType==2) //shape is a triangle
    {
        numVertices = 3; 
    }
    if (shapeType!=1) //as long as shape is NOT a circle, interpret the second constructor parameter as a list of vertices 
    {
        std::copy(vertices, vertices + numVertices, vertexArray);
    }
}

当然，与color类似。

额外建议

当然，使用std::vector或类似的代码会更好，而不是使用普通数组，使用和enum而不是int来表示shapeType等。我建议您选择一本好的C++书。