Thread: 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?

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

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...