#include <iostream>
using namespace std;
int main()
{
string s("Hello World!!");
string::size_type punct_cnt=0;
for (string::size_type index=0;index!=s.size();++index)
{
  if (ispunct(s[index]))
   ++punct_cnt;
}
cout <<punct_cnt
   <<" punctuation characters in"
   <<s
   <<endl;
return 0;
}

补上string的头文件。
#include<iostream>
#include<string>
using namespace std;

楼上正解