这是书上例题的答案
#include <iostream>
#include <string>
#include <cctype>
using namespace std;
int main()
{
    string s,result_srt;
    bool has_punct;
    char ch;
      
    cout<<"Enter a string:"<<endl;
    cin>>s;
    for (string::size_type index=0;index!=s.size();++index)
    {
        ch=s[index];
        if(ispunct(ch))
            has_punct=true;
        else
            result_srt+=ch;
    }
    if (has_punct)
    {
        cout<<"Result:"<<endl<<result_srt<<endl;
    }
    else
    {
        cout<<"No punctuation character in the string?!"<<endl;
    }
    return 0;
}
这个是我写的
#include <iostream>
#include <string>
#include <cctype>
using namespace std;
int main()
{
    string s,result_srt,ch;
    bool has_punct;
      
    cout<<"Enter a string:"<<endl;
    cin>>s;
    for (string::size_type index=0;index!=s.size();++index)
    {
        if(ispunct(s[index]))
            has_punct=true;
        else
            result_srt+=s[index];
    }
    if (has_punct)
    {
        cout<<"Result:"<<endl<<result_srt<<endl;
    }
    else
    {
        cout<<"No punctuation character in the string?!"<<endl;
    }
    return 0;
}
  想问下书上多加呢个char ch起什么作用?

多个char ch 看起来不是会好理解，代码也简洁了些

你用的方法和书上的没有本质区别，只不过它找了个替代变量。