A range based loop could be a cleaner solution:
for (const auto& i : a)
{
}
Here, i is a const reference to an element of container a.
Otherwise, if you need the index, or if you don’t want to loop over the entire range, you can get the type with decltype(a.size()).
for (decltype(a.size()) i = 0; i < a.size(); ++i) {
}