用c++统计一个英文句子中字母个数 在主程序中输入输出（用类与对象）

#include
#include 
using namespace std;
class Mystring
{
private:
    string str;
    int letterNum;
public:
    Mystring()
    {
        str = "";
        letterNum = 0;
    }
    Mystring(string s)
    {
        str = s;
        int i=0, cnt=0;
        for(i=0; i        {
            if(str[i]>='a'&&str[i]<='z'||str[i]>='A'&&str[i]<='Z')
            cnt++;
        }
        letterNum = cnt;
    }
    string getStr()
    {
        return str;
    }
    int getNum()
    {
        return letterNum;
    }
};
int main()
{
    string str;
       
    cout<<"input str:";
    getline(cin, str);
       
    Mystring ms(str);
    cout << "str=" << ms.getStr() << endl;
    cout << "字母个数：" << ms.getNum() << endl;
    system("pause");
    return 0;
}