Before You Start

Suppose I have a list of numbers, defined as

std::list<int> fibs{1,1,2,3,5,8,13,21,34,55,89};

Here are a few options that would work:

for (std::list<int>::iterator iter = fibs.begin(); iter != fibs.end(); ++iter) {
    *iter *= 2;
}

for (auto iter = fibs.begin(); iter != fibs.end(); ++iter) {
    *iter *= 2;
}

for (int& v : fibs) {
    v *= 2;
}

Is this version okay?

for (size_t i = 0; i < fibs.size(); ++i) {
    fibs[i] = fibs[i] + fibs[i];
}

Sorry, no. std::list doesn't have operator[]. But your code would work if we'd been using std::vector.
You also used operator[] three times when you could have just used it once.

Optional Extra

I've noticed that in the explicit versions, we call fibs.end() each time we go around the loop. Is that okay? Wouldn't it be inefficient?
The end() member function is constant time, so it won't make any difference to the asymptotic performance.
And end() is usually coded so it's really efficient, so the cost of calling it multiple times is negligible.
But if you really wanted, you could put the value in a variable instead.

You may see some people write their code to use an extra variable and avoid an extra end() call. For example,

for (auto iter = fibs.begin(), endpt = fibs.end(); iter != endpt; ++iter) {
    *iter *= 2;
}

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