Getting value of std::list<>::iterator to pointer?

Question

How can i loop thru a stl::List and store the value of one of the objects for use later in the function?

Particle *closestParticle;
for(list<Particle>::iterator p1 = mParticles.begin(); p1 != mParticles.end(); ++p1 )
     {
      // Extra stuff removed
            closestParticle = p1; // fails to compile (edit from comments)
     }

Answer 1

Either

Particle *closestParticle;
for(list<Particle>::iterator it=mParticles.begin(); it!=mParticles.end(); ++it)
    {
      // Extra stuff removed
            closestParticle = &*it;
    }

or

list<Particle>::iterator closestParticle;
for(list<Particle>::iterator it=mParticles.begin(); it!=mParticles.end(); ++it )
    {
      // Extra stuff removed
            closestParticle = it;
    }

or

inline list<Particle>::iterator findClosestParticle(list<Particle>& pl)
{
    for(list<Particle>::iterator it=pl.begin(); it!=pl.end(); ++it )
        {
          // Extra stuff removed
               return it;
        }
}

or

template< typename It > 
inline It findClosestParticle(It begin, It end)
{
    while(begin != end )
        {
          // Extra stuff removed
               return it;
          ++it;
        }
}

These are sorted in increasing personal preference. :)

Answer 2

For a list, the only way to invalidate an iterator is to erase it. So I suspect you're calling list.erase(p1) at some point in the loop. You need to make a copy of the iterator, move p1 back one, and then erase the copy.

EDIT: Oh wait, did you mean it doesn't compile? If so, see @sbi's answer. But you really need to word your question in a good way. What is your compile error? Or does it fail at run-time? In this case, however, I believe you mean a compile error.

Answer 3

I'm not an expert on the STL, but I believe the reason it fails to compile is because an iterator is an object that points to another object. In other words, an iterator is a generalization of a pointer. So to do what you'd want with minimal changes to your code, you would first need to de-reference the iterator to get at the value it contains. You'd then use the '&' to get its address and would then assign that address to your pointer variable. This is why ptr=&*it; works.