Re: [boost] Re: lexicographic: review request?

From: Brian McNamara (lorgon_at_[hidden])
Date: 2003-08-23 08:45:42

On Sat, Aug 23, 2003 at 11:21:03AM +0200, Daniel Frey wrote:
> a) Short-circuiting
> b) Unnamed functions
> 
> Jan Langer wrote:
> >bool operator < (person const &p1, person const &p2)
> >{
> >    return boost::lexicographic
> >        (p1.lastname, p2.lastname, cmp_lower)
> >        (p1.firstname, p2.firstname, cmp_lower)
> >        (p2.age, p1.age);
> >}
> 
> The two points listed above are both visible here: Even if there are no 
> persons with the same name, age is accessed. Even worse, it may be a 
> costly method age() and the unaware user creates much worse code than 
> using nested if-else-structures.
> 
> Also, you might not want to define operator<. Sometimes, you just need 
> it in one place, e.g. std::sort( v.begin(), v.end(), /*HERE*/ );

I have imagined another syntax which addresses the issues somewhat:

   // use boost::lamdba
   boost::lexicographic<person>
         ( _1.first,        cmp_lower )    /* see below */
         ( _1.last,         cmp_lower )
         ( bind(&T::age,_1) /* use default cmp */ )
         ( p1, p2 )

The basic idea is you say lexicographic<T> to start creating a
comparator for T objects, and then use continual argument lists (as in
the original example), but where each argument list is either
    ( lambda(T){ return T.component }, component_comparator )
or
    ( lambda(T){ return T.component } )  // uses default comparator
Finally, when you see an argument list with arguments of type
    ( T, T )
you know you are done and it is time to compare now.  Of course, if you
leave off this final argument list, then you get back a function object
which can be passed, e.g., to std::sort() as in the example above.

The problem (cited "see below") is that since operator "." isn't
overloadable, the boost::lambda expressions like "_1.first" don't
actually exist.  Offhand I dunno if C++ in general or Boost in
particular offers a nice way to lambda-ify such a member-variable
expression.

Anyway, just wanted to share some ideas which I think may be a good
direction to go to address the issues Daniel raised. 

-- 
-Brian McNamara (lorgon_at_[hidden])