c++ for loop temporary variable use

Question

Which of the following is better and why? (Particular to c++)

a.

int i(0), iMax(vec.length());//vec is a container, say std::vector
for(;i < iMax; ++i)
{
  //loop body
}

b.

for( int i(0);i < vec.length(); ++i)
{
  //loop body
}

I have seen advice for (a) because of the call to length function. This is bothering me. Doesn't any modern compiler do the optimization of (b) to be similar to (a)?

Answer 1

Example (b) has a different meaning to example (a), and the compiler must interpret it as you write it.

If, (for some made-up reason that I can't think of), I wrote code to do this:

for( int i(0);i < vec.length(); ++i)
{
    if(i%4 == 0)
       vec.push_back(Widget());
}

I really would not have wanted the compiler to optimise out each call to vec.length(), because I would get different results.

Answer 2

I like:

for (int i = 0, e = vec.length(); i != e; ++i)

Of course, this would also work for iterators:

for (vector<int>::const_iterator i = v.begin(), e = v.end(); i != e; ++i)

I like this because it's both efficient (calling end() just once), and also relatively succinct (only having to type vector<int>::const_iterator once).