如下代码，在要求输入判断字符ch时，程式自动将ch赋值为112 'p'，直接跳过，请教下各位原因！
carton* carton::createlist()
{
     carton* head=NULL;
     carton* end=head;
     carton* pstr;
     char ch=0;
     cout<<"Please input a mentral about a carton!"<<endl;
     do{
         double heightvalue=0,lengthvalue=0,widethvalue=0;
         cin>>heightvalue>>lengthvalue>>widethvalue;
         string* mentral=new string;
         getline(cin,*mentral);
         pstr=new carton(heightvalue,lengthvalue,widethvalue,*mentral);
         if(head==NULL)
             head=pstr;
         else
             end->next=pstr;
         end=pstr;
         cout<<"Do you want to input again?"<<endl;
         cin>>ch;                                     ////////此处出现错误
     }while(tolower(ch)=='y');
     end->next=NULL;
     phead=head;
     return head;
};

cin.sync();
getline(cin,*mentral);

感谢！