[boost] About member extraction

[Daryle Walker]

In another thread, by Joaqu�n M L�pez Mu�oz, there is talk of a helper 
class like:

//================================================================
template<
  class Class,typename Type,
  Type Class::*PtrToMember,
  typename Compare=std::less<Type> >
struct less_by
{
  less_by(const Compare& comp=Compare()):comp(comp){}
  bool operator()(const Class& x,const Class& y)const
  {
    return comp(x.*PtrToMember,y.*PtrToMember);
  }
  bool operator()(const Type& x,const Class& y)const
  {
    return comp(x,y.*PtrToMember);
  }
  bool operator()(const Class& x,const Type& y)const
  {
    return comp(x.*PtrToMember,y);
  }
private:
  Compare comp;
};
//================================================================
[That was cut from the "multindex" trial code.]  The author later 
decided to segregate the extraction from the comparison.  No code has 
been given yet, but that class could be like:

//================================================================
template<
  class Class,typename Type,
  Type Class::*PtrToMember >
struct member
{
    Type const &  operator ()( Class const &c ) const
        { return c.*PtrToMember; }
    Type &  operator ()( Class &c ) const
        { return c.*PtrToMember; }
};
//================================================================
But doesn't the "PtrToMember" template parameter already imply the 
"Type" and "Class" parameters?  So specifying all three would be 
redundant.  Could we reduce it by:

//================================================================
template < typename PtrToMember >
struct member_extractor
{
  // Don't know if this is a real type-traits class
  BOOST_STATIC_ASSERT(is_pointer_data_member<PtrToMember>::value);
  // The extractor traits classes aren't real (yet, maybe)
  typedef get_class_type<PtrToMember>  argument_type;
  typedef get_member_type<PtrToMember>   return_type;
  return_type const &  operator ()( argument_type const &c ) const
    { return c.*PtrToMember; }
  return_type &  operator ()( argument_type &c ) const
    { return c.*PtrToMember; }
};
//================================================================
Or we can simplify the type extraction, at the cost/gain of needing to 
specify the exact member at construction time by:

//================================================================
template < class Class, typename Type >
class member_extractor
{
public:
  typedef Class  argument_type;
  typedef Type     return_type;
  typedef Type Class::*  member_type;

  explicit member_extractor( member_type m )
    : member_( m ) {}
  return_type const &  operator ()( argument_type const &c ) const
    { return c.*member_; }
  return_type &  operator ()( argument_type &c ) const
    { return c.*member_; }
  member_type get_member() const
    { return this->member_; }
private:
  member_type  member_;
};
//================================================================
Daryle

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