Question

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.

Answer 1

I usually write this code as:

/* i and size are local to the loop */
for (size_t i = firstString.size(), size = anotherString.size(); i < size; ++i) {
  // do something
}

This way I do not pollute the parent scope and avoid calling anotherString.size() for each loop iteration.

It is especially useful with iterators:

for(some_generic_type<T>::forward_iterator it = container.begin(), end = container.end();
    it != end; ++it) {
  // do something with *it
}

Since C++ 11 the code can be shortened even more by writing a range-based for loop:

for(const auto& item : container) {
  // do something with item
}

or

for(auto item : container) {
  // do something with item
}