String类仅仅查找操作 就为我们提供了6种查找函数，每种函数以不同形式的find命名。

find、rfind、find_first_of 、find_first_not_of、find_last_of 、find_last_not_of

这些都是const成员函数，const 也就是说，它们只是为了针对某种用途去寻找子串的位置，并不能够改变它们所作用的那个串的值。

-String type only to find the operation provides us with the six kinds of search functions, each function in order to find different forms of name. find, rfind, find_first_of, find_first_not_of, find_last_of, find_last_not_of These are const member functions, const In other words, they are only for use against a substring to search for the location, they will not change the role of the string value.