Question

I have a class with two setter. One, the parameter is constant, and one is not.

class  Author
{
  string name;
  Book* book;
public:
  void setName(const string& name) { this->name = name; }  // no error
  void setBook(const Book* book) { this->book = book; } // error: value of const Book* cannot assign to Book*
}

My question is: why at setName method, I can make parameter constant with no error, but not in setBook.

Thanks 🙂

Answer 1

because you would be able to modify the object *book points to after assigning it to this->book, thus circumventing the constness of the argument.

(you’d have to explicitly cast it to a pointer to a non-const object)

In other words,

• the declaration setBook(const Book* book) means that the object pointed to by book will not be modified (that’s a ‘promise’ to the calling function).
• The field this->book is however declared in a way that you can easily modify the object pointed to by it.

Through the assignment this->book from the function argument book you would be able to later on modify the object originally pointed to by the function argument book.