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

25 Mar 2004

      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 a, pairb)
{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