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 ([EMAIL PROTECTED])
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