Re: [Boost-users] Generically comparing structs

Subject: Re: [Boost-users] Generically comparing structs
From: Bill Buklis (boostuser_at_[hidden])
Date: 2009-08-27 17:06:50

> -----Original Message-----
> From: Steven Watanabe [mailto:watanabesj_at_[hidden]]
> Sent: Thursday, August 27, 2009 1:43 PM
> To: boost-users_at_[hidden]
> Subject: Re: [Boost-users] Generically comparing structs
> 
> AMDG
> 
> Bill Buklis wrote:
> > Frequently I have to write comparison functions (usually of the "less"
> > variety) that involve multiple keys or members of a struct. For example:
> >
> > struct s
> > {
> >   A   a;
> >   B   b;
> >   C   c;
> >   D   d;
> >   // there may be other members not used in the comparison
> > };
> >
> > // operator< function or an equivalent function object
> > bool operator<( const s& lhs, const s& rhs )
> > {
> >   if( lhs.a < rhs.a )
> >      return(true);
> >   else if( lhs.a == rhs.a )
> >   {
> >      if( lhs.b < rhs.b )
> >         return(true);
> >      else if( lhs.b == rhs.b )
> > //and so on...
> >   }
> >   return(false);
> > }
> >
> > This gets quite repetitive, especially if I have to write alternate sort
> > orders (e.g. b,a,c,d or c,d,a,b)
> >
> > I'm thinking there has to an easier way to do this or a way to do a
> generic
> > algorithm for this. Is there anything in boost that would help?
> >
> > I had an idea that one might be able to fill a vector with binders for
> each
> > element and then call lexicographical_compare, but I'm not sure how to
> make
> > it work.
> >
> 
> Boost.Fusion provides several ways to do this.
> 
> #include <boost/fusion/include/less.hpp>
> #include <boost/fusion/include/adapt_struct.hpp>
> 
> BOOST_FUSION_ADAPT_STRUCT(s, (A, a)(B, b)(C, c)(D, d))
> 
> bool operator<( const s& lhs, const s& rhs )
> {
>     return boost::fusion::less(lhs, rhs);
> }
> 
> If you need different sort orders, you can use tie:
> 
> #include <boost/fusion/include/vector_tie.hpp>
> 
> bool operator<( const s& lhs, const s& rhs )
> {
>     return boost::fusion::vector_tie(lhs.a, lhs.b, lhs.c, lhs.d) <
>         boost::fusion::vector_tie(rhs.a, rhs.b, rhs.c, rhs.d);
> }
> 
> In Christ,
> Steven Watanabe
> 

OK, one more related question:

If one of the members was a C-style string, how can I override the
comparison to use strcmp or equivalent?

-- Bill --