Question

Please look at the following code and tell me if it’s going to cause problems in the future, and if yes, how to avoid them.

class Note
{
   int id;
   std::string text;

public:
   // ... some ctors here...

   Note(const Note& other) : id(other.id), text(other.text) {}

   void operator=(const Note& other) // returns void: no chaining wanted
   {
      if (&other == this) return;
      text = other.text;  
      // NB: id stays the same!    
   }
   ...
};

In short, I want the copy constructor to create an exact copy of an object, including its (database) ID field. On the other hand, when I assign, I want just to copy the data fields.
But I have some concerns, since usually the copy ctor and the operator= have same semantics.

The id field is used only by Note and its friends. For all other clients, the assignment operator does create an exact copy. The use case: When I want to edit a note, I create a copy using copy ctor, edit it, and then call save on the Notebook class which manages Notes:

 Note n(notebook.getNote(id));
 n = editNote(n); // pass by const ref (for the case edit is canceled)
 notebook.saveNote(n);

When on the other hand I want to create a completely new note with the same contents as an existing note, I can do this:

 Note n; 
 n = notebook.getNote(id); 
 n.setText("This is a copy");
 notebook.addNote(n);

Is this approach plausible? If not, please point out what the possible negative consequences are! Thanks a lot!

Answer 1

If you want semantics that don’t match what’s expected from the assignment operator, then don’t use it. Instead, disable it by declaring a private operator= and define a function with a name that makes clear what’s going on, like copyDataFields.