Help with creating a class
 

Hello everyone I have to create a class that has the following:
I would like some input if I did the above correctly.

This is what is in my header file:

class Field{

    public:

        virtual void display();

        virtual int edit();

        virtual bool editable() const;

        virtual void *data();

        virtual Field *clone() const;

};

This is what I have in my cpp file:

// Field Class functions

void Field::display(){

    

}

int Field::edit(){

    return 0;

}

bool Field::editable() const{

    return false;

}

void Field::*data(){

    return NULL;

}

void Field::*clone() const{

    return NULL;

}

I am getting errors with what I currently have and decided to break it down piece by piece. I would like to know if what I did is correct, any input is highly appreciated.

Re: Help with creating a class
 

// Star is in the wrong place

void Field::*data(){

    return NULL;

}



// Star is in the wrong place

// void* is the wrong return type

void Field::*clone() const{

    return NULL;

}

The class name and scope operator go right next to the method name, after the return type. You just need to move the star over a bit. In your class you say that clone() returns a Field* too:

void *Field::data(){

    return NULL;

}



Field *Field::clone() const{

    return NULL;

}

Make sure you include a header that defines NULL, like <cstddef>, or that might cause an error too.

On a design note, the entire Field class should be abstract because it's a base class. That saves you the need to implement dummy methods that don't do anything but return a useless value. ;) A good rule of thumb is to make any base class abstract and only leaf classes concrete. That way you don't have to worry about problems like slicing. Also, if you plan on using Field polymorphically, a virtual destructor is a good idea:

#include <iostream>

#include <string>



class Field{

public:

  virtual void display() = 0;

  virtual int edit() = 0;

  virtual bool editable() const = 0;

  virtual void *data() = 0;

  virtual Field *clone() const = 0;

  virtual ~Field() = 0 {}

};



class TextField: public Field {

  std::string _text;

public:

  TextField(std::string text): _text(text) {}

  ~TextField() {}



  void display() { std::cout << _text << '\n'; }

  int edit() { return 0; }

  bool editable() const { return false; }

  void *data() { return (void*)_text.c_str(); }

  Field *clone() const { return new TextField(_text); }

};



int main()

{

  Field *field1 = new TextField("Ed Rules!");

  Field *field2 = field1->clone();



  field1->display();

  field2->display();



  delete field2;

  delete field1;

}

Re: Help with creating a class
 

Well No Wonder You are getting errors. This is because you have declared a void function and giving a return value to it.

Re: Help with creating a class
 

Thanks for that information! Although when I compile that example program I get an error saying "pure specifier on function definition" on this line:

virtual ~Field() = 0 {}

Re: Help with creating a class
 

Sorry, Edward was accidentally using a Visual Studio extension. This should work:

#include <iostream>

#include <string>



class Field{

public:

  virtual void display() = 0;

  virtual int edit() = 0;

  virtual bool editable() const = 0;

  virtual void *data() = 0;

  virtual Field *clone() const = 0;

  virtual ~Field() = 0;

};



Field::~Field() {}



class TextField: public Field {

  std::string _text;

public:

  TextField(std::string text): _text(text) {}

  ~TextField() {}



  void display() { std::cout << _text << '\n'; }

  int edit() { return 0; }

  bool editable() const { return false; }

  void *data() { return (void*)_text.c_str(); }

  Field *clone() const { return new TextField(_text); }

};



int main()

{

  Field *field1 = new TextField("Ed Rules!");

  Field *field2 = field1->clone();



  field1->display();

  field2->display();



  delete field2;

  delete field1;

}

Re: Help with creating a class
 

I see thanks!