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

From: Duane Murphy (duanemurphy_at_[hidden])
Date: 2004-03-25 19:58:09

--- At Fri, 26 Mar 2004 00:48:49 +0100, Daniel Wallin wrote:

>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)
>       )
>   );

I don't know lambda, so I won't comment on that. But, I thought that the
first parameter to bind was a function pointer? You can bind to an address? 

Please explain how this works?

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

 ...Duane