Question

std::set<std::string> setStrings;

setString.insert("abc");
setString.insert("abcd");
setString.insert("babc");

Question> I would like to know how to check whether “bab” is one of the stored string’s prefix?

Answer 1

I assume you don’t want to search the entire set: just use std::set<std::string>::lower_bound() and iterate until you find a std::string which doesn’t have the desired prefix:

std::string const prefix("bab");
for (std::set<std::string>::const_iterator it(setStrings.lower_bound(prefix));
     it != setStrings.end() && it->find(prefix) == 0; ++it) {
    std::cout << "prefixed: '" << *it << "'\n";
}

If you just want to find if there is one string with the corresponding prefix you can use the condition as in the loop.