TITLE: how to sort a std::list of pointers

(Newsgroups: comp.lang.c++, comp.lang.c++.moderated, 27 Jan 2000)

KULKARNI: "Yogesh Kulkarni" <yogi_kulkarni@yahoo.com>

> I have a std::list of pointers to foo, where foo is my class. Because 
> I want to sort the list based on some member of foo, I provided 
> operator< in the foo, which I thought, would enable the sorting. What 
> I found out later is that the list was not getting sorted based on 
> the foo's data member values. It was in fact gettng sorted on the 
> memory addresses where the pointers in the list were allocated. I am 
> sure, this technique does work on list of solid objects, but is 
> apparently not appropriate for the list of pointers. The question is, 
> how can I provide a custom sort mechanism that enables me to sort the 
> list of pointers based on the values of objects that are pointed to 
> by the pointers.

ADLER: Darin Adler <darin@bentspoon.com>

You need to define a function object. Here's how I'd do it.

   template <typename T>
   struct LessByPointedToValue
      : std::binary_function<T const *, T const *, bool>
   {
      bool operator()(T const * x, T const * y) const
         { return std::less<T>()(*x, *y); }
   };

   // ...
   std::list<foo*> fl;
   // ...
   fl.sort(LessByPointedToValue<foo>());
   // ...

The call to sort passes a function object, an instance of the 
class<LessByPointedToValuefoo>, which is an instance of the class 
template LessByPointedToValue.



_______________________________________________
cpptips mailing list
http://cpptips.hyperformix.com