Why does the content of a pointer change? - c++

Question

I'm a C# programmer and now using C++ to do some work.
pair<Point,double>* p=NULL;
Sphere* sphere=NULL;
for (int i=0;i<spheres.size();i++)
{
vector<pair<Point,double>> solution=findIntersection(Point(ray.origin),Point(ray.direction.x,ray.direction.y,ray.direction.z),spheres[i]);
if(solution.size()==0)
continue;
if(p==NULL || solution[0].second<p->second)
{
p=&solution[0];
sphere=&spheres[i];
}
}
if(p==NULL)
return backgroundColor;
else
{
Color c=localIlluminate(p->first,*sphere);
return c;
}
I want p.first to have the smallest value, and sphere be the cooresponding sphere that is used to get p.
After debugging, I find the code doesn't work. In the first loop, p will be assigned the address of solution[0], assuming the value is {(0,0),0}. Then the loop continues and when i=1, assume solution[0] becomes {(1,2),3} and value of p also becomes {(1,2),3}.
I don't expect the value of p to change. How should I fix it?

Answer 1

You are storing the reference to a local variable outside the scope in which the local variable is declared.
Every iteration solution is not valid anymore, then the address to it shouldn't be considered valid. To obtain what you need you should assign the variable by value, so that you actually copy the contained value, eg:
pair<Point, double> p = std::make_pair(whatever, std::numeric_limits<double>::max());
for (...)
{
if (solution[0].second < p.second)
p = solution[0];
}
The fact that the address changes can be caused by multiple reasons, but you shouldn't bother understanding why, just avoid this kind of situation. Your misconception comes from the fact that C# has a garbage collection which prevents solution[0] from becoming invalid, which is not true for C++, when variable are declared on stack.

Answer 2

When you assign to p it points to the adresss of &solution[0] but on the next iteration of the loop that variable goes poof and a new one gets created and p points to either random stuff or something else.
It might be better to just store a copy in p, so make p a regular variable and copy over the solution[0] by assigning. You can have another bool variable to determine if a solution was found.
pair<Point,double> p;
Sphere sphere;
bool solutionFound = false;
for (int i=0;i<spheres.size();i++)
{
vector<pair<Point,double>> solution=findIntersection(Point(ray.origin),Point(ray.direction.x,ray.direction.y,ray.direction.z),spheres[i]);
if(solution.size()==0)
continue;
if(!solutionFound || solution[0].second < p.second)
{
p=solution[0];
sphere=spheres[i];
solutionFound = true;
}
}
if(!solutionFound)
return backgroundColor;
else
{
Color c=localIlluminate(p.first, sphere);
return c;
}