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;
        }
    return pl.end();
}

or

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

These are sorted in increasing personal preference. :)