Re: [Boost-users] Question on ptr_vector class

From: Thorsten Ottosen (tottosen_at_[hidden])
Date: 2006-03-21 11:57:57

Sebastien Fortier wrote:

 > I would like to define two different sorting predicates for a class
 > but I do not understand how to do this.
 >
 > //meteorlogicalObj.hpp
 > class meteorlogicalObj {
 > public:
 >    meteorlogicalObj(int val, Point pt)    : val_(val), pt_(pt){}
 >    virtual      ~meteorlogicalObj() {}
 >
 >    // how would I go about creating these sorting functions that 
would work with the ptr_vector class
 >    bool sortOnValue(const meteorlogicalObj&, const meteorlogicalObj&);
 >    bool sortOnPoint(const meteorlogicalObj&, const meteorlogicalObj&);
 >    // ...
 > private:
 >    int val_;
 >    Point pt_;
 > };
 >
 > //meteorlogicalObj.cpp
 > #include "meteorlogicalObj.h"
 >
 > bool meteorlogicalObj::sortOnValue(const meteorlogicalObj& metObj, 
const meteorlogicalObj& other)
 > {
 >    return (metObj.val_ < other.val_);
 > }
 >
 >
 > bool meteorlogicalObj::sortOnPoint(const meteorlogicalObj& metObj, 
const meteorlogicalObj& other)
 > {
 >    return (metObj.pt_ < other.pt_);
 > }
 >
 > //main.cpp
 > boost::ptr_vector<meteorlogicalObj>  metObjVec;
 >
 > //... fill vector
 >
 > metObjVec.sort( sortOnValue() ); // this does not work, what is 
missing???
 > metObjVec.sort( sortOnPoint() ); // this does not work, what is 
missing???
 >
 > I can see that I probably need the indect_fun but I do not know how 
to wrap my sorting function to pass them to the sorting function

You don't need to use indirect_fun<>, the algorithms do that themselves.

What you is a functor that can be properly contructed:

struct SortOnValue
{
   typedef bool result_type;
   template< class M >
   bool operator()( const M& l, const M& r ) const
   { return l.value() < r.value(); }
};

I assume that you have added a function called value to 
meteorlogicalObj. Alternatively you can make the class SortOnValue a 
friend to the class.

-Thorsten