What is the preferred method of writing loops according to efficiency: Way a)

   /*here I'm hoping that compiler will optimize this  
 code and won't be calling size every time it iterates through this loop*/
    for (unsigned i = firstString.size(); i < anotherString.size(), ++i)
    {
    //do something
    }

or maybe should I do it this way: Way b)

unsigned first = firstString.size();
unsigned second = anotherString.size();

and now I can write:

    for (unsigned i = first; i < second, ++i)
    {
    //do something
    }

the second way seems to me like worse option for two reasons: scope polluting and verbosity but it has the advantage of being sure that size() will be invoked once for each object.
Looking forward to your answers.