whileループ（C++プログラム）のクロス初期化エラー

Question

ここでは多くのことを間違っています。 Plzヘルプ。

変数 stが初期化が、使用されていません

st->readdata(); 

：

#include <iostream> 
#include <fstream> 

using namespace std; 

class student { 
    int rno; 
    char name[15]; 
    float marks; 

public: 
    void getdata() 
    { 
     cout << "\nEnter Roll No. : "; 
     cin >> rno; 

     cout << "\nEnter Name : "; 
     cin >> name; 

     cout << "\nEnter Marks : "; 
     cin >> marks; 
    }; 

    void writedata() 
    { 
     fstream input; 
     input.open("stu.dat", ios::out | ios::app); 
     input.write((char*)this, sizeof(student)); 
     input.close(); 
    }; 

    void readdata() 
    { 
     int temp; 
     cout << "\nEnter roll no: "; 
     cin >> temp; 

     fstream output; 
     output.open("stu.dat", ios::in); 
     output.seekg(0, ios::beg); 

     while (output.read((char*)this, sizeof(student))) { 
      if (rno == temp) { 
       cout << "\nRoll no. : " << rno 
        << "\nName : " << name 
        << "\nMarks : " << marks; 
      } 
      else { 
       cout << "\nWrong Roll no entered"; 
      }; 
     }; 
    }; 
}; 

int main() 
{ 
    student s[25]; 
    student* st; 

    char ans = 'y', ans2 = 'y', ans3 = 'y'; 

    int o, i = 0, j = 0; 

    do { 
     cout << "\nEnter your choice\n1.Write\n2.Read"; 
     cin >> o; 

     switch (o) { 
     case 1: 
      while (ans2 == 'y' || ans2 == 'Y') {  
       s[i].getdata(); 
       s[i].writedata(); 

       cout << "\nDo you want to continue?";  
       cin >> ans2; 

       i++; 
      }; 
      break; 
     case 2: 
      while (ans3 == 'y' || ans3 == 'Y') { 
       st->readdata(); 
       cout << "\nDo you want to continue?"; 
       cin >> ans3; 

       j++; 
      }; 

      break; 
     }; 
    } while (ans == 'y' || ans == 'Y'); 

    return 0; 
} 

Answer 1

すべてがライン82までもそうです。