Split String to Vector with Whitespaces c++ error

#include <iostream>
#include <string>
#include <vector>
#include <sstream>

using namespace std;


vector<string> split_string(string s)
{
string buf;
stringstream ss(s);

vector<string> tokens;

while (ss >> buf)
    tokens.push_back(buf);

return tokens;
}


int main()
{
   cout << split_string("Alpha Beta Gamma");
}
#include <iostream>
#include <string>
#include