求字符串长度 总是说 ch 没有定义，我定义了结果错得更多了，该怎么办？ 帮看看吧！

#include<iostream>
#include<string>
using namespace std;
class strtype
{
public:
    string  ch;
    int show();
    const strtype& operator =(const string &str)
    {
        ch = str;
        return *this;
    }
};
int strtype::show()
{
    int len = 0;
    int i = 0;
    while (ch[i++] !=  '\0')
    {
        len++;
    }
    cout << "字符串的长度" << len << endl;
    return 0;
}
int main()
{
    strtype *p;
    p = new strtype;
    cout << "Creating string" << endl;
    p->ch = "we study new and delete";
    cout << "Deleting string" << endl;
    p->show();
    cout << p->ch << endl;
    system("pause");
    return 0;
}