C++ STL : Use of functional object and a tip [2006/07/15]

STL is convienient to use and has much benifits over robustness and performance. For algorithms like sorting and priority queue, we need comparision function to decide order among elements . It is easy to do so when the element is our own class. We need only write the less operator. like this

class foo
{
.....
`  bool operator < (const foo& _right)    {        return value <>

Then when you sort a vector e.g.  vector vf, it will be sorted according to value.

When we use standard containers like or types, we don't have chance to override the less function.
then. we can use functional object. (an object act as  function)  here are examples.
1.   define a function object for the object to sort.
 

   class fobj_comp{
         public:
        bool operator () (const foo& _p1, const foo& _p2) const
        {
             return _p1.another_value <>
 and supply it to sort routine  sort(vf.begin(), vf.end(), fobj_comp());
 so that we can sort the vector on "another_value" without need to modify the inside "<" operator.  2. Use functional object  for dereferencing " pair*"

  class fobj_comp{
     public:
     bool operator () (const pair* _p1, const pair* _p2) const
     {
          return _p1->first <>first;
     }
 }

and then define the prioirty queue as

priority_queue <>* , vector* > , fobj_comp>  my_pq;

so that priority function is defined as fobj_comp;

The EFF_STL book suggests base such fobj_comp on

binary_function<>*, pair* >


changing it to

class fobj_comp :
public binary_function<>*, pair* >
{
     //same as previous body
}


They have the same effect for the code segment.

One point is in operation def. We can not write

 bool operator () (const foo*& _p1, const foo*& _p2) const
 {
         .....
 }

The compiler can not read "foo*&"  use "foo*" or typedef foo* pfoo, and const pfool& works

[Thanks]
 Jency for EFF_STL,  Shu'an for finding the last point. , Mr. google
 Ref.  Effect_STL book. (ask Mr. google for details)