解决方法
Boost：
http://www.boost.org/doc/libs/1_54_0/doc/html/boost/algorithm/find_nth.html中有一个find_nth模板函数
#include <iostream>
#include <boost/algorithm/string/find.hpp>
using namespace std;
using namespace boost;
int main() {
string a = "The rain in Spain falls mainly on the plain";
iterator_range<string::iterator> r = find_nth(a,"ain",2);
cout << distance(a.begin(),r.begin()) << endl;
return 0;
}
