Re: [glob2-devel] question about std::list

[Bradley Arsenault]

On 5/2/07, Leo Wandersleb <address@hidden> wrote:
> hi
>
> fighting to get a meaningful unit allocation i ran into a sort that doesn't 
> sort:
> ok it sorted. but it sorted pointers. not what i thought it would sort.
>
> ok now that i know and that it works, can someone tell me how to do it clean?
>
> i had
> unitsWorking.sort();
> with
> public:
> Sint32 score;
> bool operator < (const Unit& u) { return score < u.score; }
> in Unit.h
> and that was only sorting adresses.
>
> i found out that:
> class Predicate
> {
>         public:
>         bool operator() (const Unit* lhs, const Unit* rhs) {
>                 return lhs->score < rhs->score;
>         }
> };
> and then
> unitsWorking.sort(Predicate());
> worked as expected. But how can i tell it to use my operator <??
>
> thanx for input
>
> Leo Wandersleb
>
> P.S.: Unit Alocation still is fun to work at although i'm not too impressd by my
> success :/

Agreed, use std::list::sort rather than std::sort as it has a
customized algorithm.

Second of all, you need to pass in the function,somehow. Just calling
unitsWorking.sort() will sort the data in unitsWorking, which is
sorting the pointers, as you so described.

Making a  bool operator < (const Unit& u) { return score < u.score; }
in Unit.h won't work either, because the data in the list are
pointers, Unit*, not references, Uint& or Unit. Doing a comparison
function in unit.h that operates with pointers won't work either, as
its still not qualified.

> From my learnings, the only truly clean way to sort a list of pointers
based on the pointers contents, rather than the pointer itself, is
through a predicate as follows:


template<typename t1> pointerCompare(t1* lhs, t1* rhs) { return (*lhs)
< (*rhs); };
and passing that into sort like:
unitsWorking.sort(pointerCompare<Unit>);

Thats about as clean as possible, although a more meaningfull name
than pointerCompare may be nesseccarry.



--
Really. I'm not lieing. Bradley Arsenault.