Question

I’m getting a Segmentation Fault when trying to call a function that is within a object that is part of a vector of “Shape” Pointers

My problem is in this function::

    Point findIntersection(Point p, Point vecDir, int *status)
    {

        Point noPt;
        for (int i = 0; i < shapes.size(); i++)
        {
            Point temp;
            cout << "Shapes size" << shapes.size() << endl;
**SEGMENTATIONFAULT HERE >>>>>**            bool intersect = shapes[0]->checkIntersect(p, vecDir, &temp);
            if (intersect)
            {
                *status = 1;    // Code 1 for intersecting the actual shape
                return temp;
            }

        }

        return noPt;
    }

Initially, I am only adding one shape:

void createScene()
{

    image = QImage(width, height, 32); // 32 Bit

    Sphere s(Point(0.0,0.0,-50.0), 40.0);
    shapes.push_back(&s);
    cout << shapes.size() <<endl;
}

So I have a vector of “shapes” which is global.
vector shapes;

I have a class shape

#include "Point.h"
#ifndef SHAPE_H
#define SHAPE_H
using namespace std;
class Shape
{
    public: 
    Shape() {}
    ~Shape(){}
    virtual bool checkIntersect(Point p, Point d, Point *temp) {};  // If intersects, return true else false.
    virtual void printstuff() {};

};
#endif

and also a Sphere class

#include "shape.h"
#include <math.h>
#include <algorithm>
using std::cout;
using std:: endl;
using std::min;

class Sphere : public Shape
{
    public:
    Point centerPt;
    double radius;

    Sphere(Point center, double rad)
    {
        centerPt = center;
        radius = rad;
    }

    bool checkIntersect(Point p, Point vecDir, Point *temp)
    {
        cout << "Hi" << endl;
    /*
        Point _D = p - centerPt;
        double a = Point :: dot(vecDir, vecDir);
        double b = 2 * ( Point :: dot(vecDir, _D) );
        double c = (Point :: dot(_D,_D)) - (radius * radius);

        // Quadratic Equation
        double tempNum = b * b - 4 * a * c;
        if (tempNum < 0)
        {
            return false;
        } else
        {
            double t1 = ( -b + sqrt(tempNum) ) / (2 * a);
            double t2 = ( -b - sqrt(tempNum) ) / (2 * a);
            double t;

            if (t1 < 0 && t2 > 0) { t = t2; }
            else if (t2 < 0 && t1 > 0) { t = t1; }
            else if ( t1 < 0 && t2 < 0 ) { return false; }
            else
            {
                t = min(t1, t2);
            }

            Point p1 = p + (vecDir * t);

            if (p1.z > 0)           // Above our camera
            {
                return false;
            } else 
            {
                temp = &p1;
                return true;
            }

        }
    */
        return false;
    }
};

Answer 1

The problem is here:

Sphere s(Point(0.0,0.0,-50.0), 40.0);
shapes.push_back(&s);

at this point, you’ve created the Sphere s locally on the stack, and you’ve pushed its address into your vector. When you leave scope, local objects are freed, and so the address you’ve stored in your vector now points to memory you no longer own and its contents is undefined.

Instead, do

Sphere *s = new Sphere(Point(0.0,0.0,-50.0), 40.0);
shapes.push_back(s);

to allocate the Sphere from the heap so that it persists. Make sure to delete it when you are done with it.