[boost] Interest in an 'either' library?

21 Jun 2013

      **semantics**:

template< typename left_type, typename right_type>
struct either
{
  // postcondition, is_left()
  either( left_type );

  // postcondition, is_right()
  either( right_type );

  bool is_left() const;
  bool is_right() const;

  // postcondition, is_left()
  either & operator=( left_type );

  // postcondition, is_right()
  either & operator=( right_type );

  // precondition, is_left()
  left_type left();

  // precondition, is_right()
  right_type right();
private:
};

**uses**:

**Use 1**: Can be used as an alternative to exceptions or the (error
codes+set reference idiom):

either<error_code, file_id> load_file(...);

vs.

error_code load_file( file_id &, ... );

or

// throws std::exception
file_id load_file(...);

**Use 2**: A lightweight alternative to boost::variant when only 2
components are required (either relates to variant in a similar way that
pair relates to tuple ).

variant<A,B> eitherAOrB;
if( A * aPtr = boost::get<A>( &eitherAOrB ) )
  std::cout << "A: " << *aPtr;
else
  std::cout << "B: " << boost::get<B>( eitherAOrB );

vs.

either<A,B> eitherAOrB;

if( eitherAOrB.is_left() )
  std::cout << "A: " << eitherAOrB.left();
else
  std::cout << "B: " << eitherAOrB.right();

--David Sankel