In this section, we will write a helper function that identifies surrounding white space in a string and returns a copy without that, and then we are going to test it briefly.

1. As always, the header includes and using directive come first:

      #include <iostream>
      #include <string>
      #include <algorithm>
      #include <cctype>

      using namespace std;

2. Our function to trim whitespace surrounding a string takes a const reference to an existing string. It will return a new string without any surrounding whitespace:

      string trim_whitespace_surrounding(const string &s)
      {

3. The std::string provides two handy functions, which help us a lot. The first is string::find_first_not_of, which accepts a string containing all the characters we want to skip over. This is, of course, whitespace, meaning the characters space ' ', tab 't', and new line, 'n'. It returns us the first non-whitespace character position. If there is only whitespace in the string, it returns string::npos. This means that there is only an empty string left if we trim whitespace from it. So, in such a case, let's just return an empty string:

          const char whitespace[] {" tn"};
          const size_t first (s.find_first_not_of(whitespace));
          if (string::npos == first) { return {}; }

4. We know now where the new string has to begin, but we don't yet know where it has to end. Therefore, we use the other handy string function string::find_last_not_of. It will return us the last character position in the string which is no whitespace:

          const size_t last (s.find_last_not_of(whitespace));

5. Using string::substr, we can now return the part of the string, which is surrounded by whitespace but without the white space. This function takes two parameters--a position in the string to begin with and the number of characters after this position:

          return s.substr(first, (last - first + 1));
      }

6. That's it. Let's write a main function in which we create a string that surrounds a text sentence with all kinds of whitespace, in order to trim it:

      int main()
      {
          string s {" tn string surrounded by ugly"
                    " whitespace tn "};

7. We print the untrimmed and trimmed versions of the string. By surrounding the string with brackets, it's more obvious which whitespace belonged to it prior to trimming:

          cout << "{" << s << "}n";
          cout << "{" 
               << trim_whitespace_surrounding(s) 
               << "}n";
      }

8. Compiling and running the program yields us the output we expected:

      $ ./trim_whitespace 
      {  
        string surrounded by ugly whitespace    
         }
      {string surrounded by ugly whitespace}