In this section, we used string::find_first_not_of and string::find_last_not_of. Both functions accept a C-style string, which acts as a list of characters that should be skipped while searching a different character. If we have a string instance that carries the string, "foo bar", and we call find_first_not_of("bfo ") on it, it will return us the value 5, because the 'a' character is the first one that is not in the "bfo " string. The order of the characters in the argument string is not important.

The same functions exist with inverted logic, although we did not use them in this recipe: string::find_first_of and string::find_last_of.

Similar to iterator based functions, we need to check if these functions return an actual position in the string or a value that denotes that they did not find a character position fulfilling the constraints. If they did not find one, they return string::npos.

From the character positions we retrieved from these functions in our helper function, we built us a substring without surrounding whitespace, using string::substring. This function accepts a relative offset and a string length and then returns a new string instance with its own memory, which contains only that substring. For example, string{"abcdef"}.substr(2, 2) will return us a new string "cd".