trouble with fstream and polymorphism

[willynilly]

I'm trying to output the data onto a text file
Im using polymorphism(virtual and abstract class)

  #include <iostream>
#include <fstream>


using namespace std;

class Enemy{
      protected:
              int attackPower;
                   
              
               
      public:
           virtual  void attack()=0;
           
           ofstream myfile; 
           myfile.open ("Enemy.txt"); // Keep getting an error
           myfile<< "This is a line" ;
           myfile.close(); 
           
           
};



class Ninja: public Enemy{
      public:
             void attack()
             {
                  cout<<" Ninja Chop "<<endl;     
             }
};

class Monster: public Enemy{
      public:
             void attack()
             {
                  cout<<" Monster attack "<<endl;     
             }      
      
};

int main()
{
    
    Ninja n;
    Monster m;
    Enemy *enemy1 = &n;
    Enemy *enemy2 = &m;
    enemy1 -> attack();
    enemy2 -> attack();
    
    system("pause");
    return 0;
}

I need the output to be shown on a text file.

[LB]

Lines 16 to 19 have to be in a function, you can't just write arbitrary code inside a class like that.

[willynilly]

Oh so, it could be a function within a class.
Okay now that its settled, I want to get the output
from the command prompt and also have it in the
text file, would I just do;

// Function
 ofstream myfile; 
 myfile.open ("Enemy.txt");
 myfile<< attack() ;
 myfile.close();