Re: [Boost-users] Defining less on member variables/functions

From: Daniel Wallin (dalwan01_at_[hidden])
Date: 2004-03-25 18:48:49

Chris Jurney wrote:

> I've had this same pattern come up 4 or 5 times now, and each time I've 
> been unable to find a stl + boost solution that allowed me to do what I 
> want without writing my own less function.  Basically, I want something 
> that will let me do a sort, or define a map on structs or pairs without 
> writing a less that compares the member variable or .first.  
> <snip>
> No such luck.  It looks like I can do operator * and & on lambda 
> objects, but not access members through them.  Is there anything in 
> boost, stl, or elsewhere that lets me not write this:
> 
>  
> 
> class myLess
> 
> {
> 
>     bool operator() (pair<float, float> a, pair<float, float>b)
> 
>         {return a.first < b.first;}
> 
> }

Yes:

   struct X
   {
       int a;
   };

   std::vector<X> v;

With boost.bind:

   std::sort(
       v.begin()
     , v.end()
     , boost::bind(
           std::less<int>()
         , boost::bind(&X::a, _1)
         , boost::bind(&X::a, _2)
       )
   );

and with boost.lambda:

     std::sort(
         v.begin()
       , v.end()
       , bind(&X::a, _1) < bind(&X::a, _2)
     );

HTH,
-- 
Daniel Wallin