Question

does it take constant time to move the iterator to elements of string in following:

std::string str // string of size 100 MB
std::string::iterator iter = str.begin();
std::advance(iter, str.size()-1);

would it take constant time as in searching by index?

char c = str[str.size()-1];

Answer 1

Yes, that’s correct. This is guaranteed by the C++ standard (§24.3, Iterator operations):

Since only random access iterators
provide + and – operators, the library
provides two function templates
advance and distance. These function
templates use + and – for random
access iterators (and are, therefore,
constant time for them);