Overloading ==

This example is from 'You can program in C++' by Francis Glassborow, but doesnt seem to work.

Code:

#include <iostream>
#include <istream>
#include <ostream>
#include <string>
class isbn10{
public:
  isbn10( );
  explicit isbn10(std::string const &);
  ~isbn10( );
  bool is_valid( )const;
  void send_to(std::ostream & = std::cout)const;
  void get_from(std::istream & = std::cin);
  bool is_equal(isbn10 const &)const;
  inline bool operator==(isbn10 const & lhs, isbn10 const & rhs){
    return lhs.is_equal(rhs);
  }
private:
  std::string isbn_;
};

isbn10.h:14: error: ‘bool isbn10:perator==(const isbn10&, const isbn10&)’ must take exactly one argument

How to fix it?

Change "inline bool operator==" to "friend inline bool operator==". As a friend function, operaor== takes two arguments. As a member function, it takes one argument (the current object, this, is the implicit first argument).

It works perfectly now. Thanks.

or even better, don't make it a friend at all and make it a standalone function (since it doesn't need to access any private data in the class), otherwise it adds dependencies.

Prefer writing nonmember nonfriend functions: http://www.ubookcase.com/book/Addiso...3586/ch44.html

don't make it a friend at all and make it a standalone function (since it doesn't need to access any private data in the class)

Good catch on the implementation not needing friend access, though friend functions are standalone functions, at least if "standalone" means "free".

Another good thing about making it an external function is that you can have things like this:

Code:

bool operator==( const string& lhs, const string& rhs )
{
    return lhs.compare(rhs);
}

bool operator==( const string& lhs, const char* rhs )
{
    return lhs.compare(rhs);
}

bool operator==( const char* lhs, const string& rhs )
{
    return rhs.compare(lhs);
}

Then you can compare different types no matter which side of the == they're on. If it was a member function, the string example above would only be able to compare: string == string, and string == char*, but not char* == string.

There's no need to write a specialized operator==() for this class, because all data members are held by value.

>> There's no need to write a specialized operator==() for this class, because all data members are held by value.
operator== is not generated by default, so you do have to provide it if you want that functionality. Perhaps you are thinking of operator=.

Also, cpjust, in your example compare is returning 0 when the values are equal, which means your operator== returns true if the strings are not equal.

in your example compare is returning 0 when the values are equal

Doh! I knew there was something I was forgetting...

Originally Posted by Daved

>> There's no need to write a specialized operator==() for this class, because all data members are held by value.
operator== is not generated by default, so you do have to provide it if you want that functionality. Perhaps you are thinking of operator=.

Yes, I spaced on that one. 72 hours without meaningful sleep will do that to ya...