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 Thank you for your assistance!