突然，对于特定对象的一个特定函数，在c++中获取空指针

Question

以下是代码片段

Obj *temp;
temp = new Sphere(center, radius);
struct color c = {1, 0, 0};
temp->setColor(c); //works fine
temp->setShine(1); //works fine
temp->setCoefficients(0.4, 0.2, 0.2, 0.2); //works fine
temp->draw(); //NULL pointer!!
objects.push_back(temp);

temp = new Floor(1000, 20);
temp->draw(); //works fine too!
objects.push_back(temp);

其中Obj是基类，Sphere和Floor是派生类，draw是虚拟方法。

在此之前，我的绘图函数工作得很好，并且没有指针问题。突然，我收到了这个exc-bad-access错误，我调试后发现，无论我在哪里调用Sphere::draw()，" this“指针都会变为空。

(我在这里无法正确粘贴Sphere::draw()函数。)

glPushMatrix();
{
    glColor3f(c.r, c.g, c.b);
    glTranslatef(referencePoint.x, referencePoint.y, referencePoint.z);
printf("inside sphere draw\n");
    double radius = length;
    struct point points[1000][1000];
    int stacks = 1000, slices = 1000;
    int i,j;
    double h,r;
    //generate points
    for(i=0;i<=stacks;i++)
    {
        h=radius*sin(((double)i/(double)stacks)*(pi/2));
        r=radius*cos(((double)i/(double)stacks)*(pi/2));
        for(j=0;j<=slices;j++)
        {
            points[i][j].x=r*cos(((double)j/(double)slices)*2*pi);
            points[i][j].y=r*sin(((double)j/(double)slices)*2*pi);
            points[i][j].z=h;
        }
    }
    //draw quads using generated points
    for(i=0;i<stacks;i++)
    {
        glColor3f(1.0, 0, 0);
        //glColor3f((double)i/(double)stacks,(double)i/(double)stacks,(double)i/(double)stacks);
        for(j=0;j<slices;j++)
        {
            glBegin(GL_QUADS);
            {
                //upper hemisphere
                glVertex3f(points[i][j].x,points[i][j].y,points[i][j].z);
                glVertex3f(points[i][j+1].x,points[i][j+1].y,points[i][j+1].z);
                glVertex3f(points[i+1][j+1].x,points[i+1][j+1].y,points[i+1][j+1].z);
                glVertex3f(points[i+1][j].x,points[i+1][j].y,points[i+1][j].z);
                //lower hemisphere
                glVertex3f(points[i][j].x,points[i][j].y,-points[i][j].z);
                glVertex3f(points[i][j+1].x,points[i][j+1].y,-points[i][j+1].z);
                glVertex3f(points[i+1][j+1].x,points[i+1][j+1].y,-points[i+1][j+1].z);
                glVertex3f(points[i+1][j].x,points[i+1][j].y,-points[i+1][j].z);
            }glEnd();
        }
    }

}glPopMatrix();

}

Answer 1

指针不是空的，你只是试着写一些超出数组边界的东西。这里：

for(i=0;i<=stacks;i++)

这里：

for(j=0;j<=slices;j++)

这个数组的最后一个元素是pointsstacks-1，在您的循环条件中，您试图在最后一次迭代中引用pointsstacks。

此外，在这些代码行中还有另一个问题：

这里：

glVertex3f(points[i][j+1].x,points[i][j+1].y,points[i][j+1].z);
glVertex3f(points[i+1][j+1].x,points[i+1][j+1].y,points[i+1][j+1].z);
glVertex3f(points[i+1][j].x,points[i+1][j].y,points[i+1][j].z);

还有这里：

glVertex3f(points[i][j+1].x,points[i][j+1].y,-points[i][j+1].z);
glVertex3f(points[i+1][j+1].x,points[i+1][j+1].y,-points[i+1][j+1].z);
glVertex3f(points[i+1][j].x,points[i+1][j].y,-points[i+1][j].z);

这些[i+1]、[j+1]部件正试图在上一次迭代中执行与上述相同的操作。