Pointer to member functions - C++ std::list sort

Question

How do i pass a pointer to a member function to std::list.sort()?

Is this possible? Thanks

struct Node {
       uint32_t ID;
       char * Value;
};

class myClass {
          private:
            uint32_t  myValueLength;
          public:
            list<queueNode *> MyQueue;
            bool compare(Node * first, Node * second);
            bool doStuff();
}

bool myClass::compare(Node * first, Node * second) {
    unsigned int ii =0;
    while (ii < myValueLength)
    {
     if (first-> Value[ii] < second-> Value[ii]) 
     {
      return true;
     } else if (first-> Value[ii] > second-> Value[ii])
     {
      return false;
     }

     ++ii;
    }

    return false;
}

bool myClass::doStuff()
{
    list.sort(compare);
}

I want to use a length variable from within the class instead of doing strlen() within the compare function (The Value will always be the same length)

Edit: The myValueLength was not the only variable i wanted to access from within the comparison function I just simplified it to make the example shorter.

Answer 1

You may want to use a functor.

http://www.newty.de/fpt/functor.html

Answer 2

Note that std::list sorts the element according to operator< defined for that element. You need to change your compare function to use a global operator< defined for Node objects:

bool operator<(Node const& first, Node const& second) {
unsigned int ii =0;
while (ii < length)
{
    if (first.Value[ii] < second.Value[ii]) 
    {
            return true;
    } else if (first.Value[ii] > second.Value[ii])
    {
            return false;
    }

    ++ii; 
}

return false;

}

A suggested improvement will be:

bool operator<(Node const& first, Node const& second) {
    for (size_t ii =0; first.Value[ii] == second.Value[ii]; ++ii) ; // note ;
    return (first.Value[ii] < second.Value[ii]);
}

If char *Value really represents a C-style string, and you want lexicographic sorting, further improvements are possible:

bool operator<(Node const& first, Node const& second) {
     return (strcmp(first.Value, second.Value) < 0);
}

and if they are really strings, I suggest you use std::string and you can write:

bool operator<(Node const& first, Node const& second) {
     return first.Value < second.Value;
}

Answer 3

It is possible. Did you consider using boost::function?

list.sort( boost::bind( &myClass::compare, this, _1, _2 ) );

Is your 'compare' function will rely on this data? If not - you may simpy make 'compare' function to be static. And then it will be

list.sort( &myClass::compare );

You can add helper struct to do your comparison and then

list.sort( Comparer( myValueLength ) );

struct Comparer
{
    Comparer( uint32_t myValueLength ):
        length( myValueLength )
    {}

    bool operator() (Node * first, Node * second)
    {
        unsigned int ii =0;
        while (ii < length)
        {
            if (first-> Value[ii] < second-> Value[ii]) 
            {
                    return true;
            } else if (first-> Value[ii] > second-> Value[ii])
            {
                    return false;
            }

            ++ii;
        }

        return false;
    }


    uint32_t length;
};

Answer 4

Elaborating on grieve's response, why not use a functor? E.g.:

struct Functor
{
  bool operator()( char * a, char * b )
    { return strcmp(a,b) < 0; }
};

Then you could just use:

Functor f;
myList.sort(f);

You could even use your class as the Functor by defining operator()...

class myClass {
  ...
  bool operator()( queueNode * a, queueNode * b )
  { return compare( a, b ); }

  void doStuff() { MyQueue.sort(*this); }
};

---

Simple example code:

#include <iostream>
#include <list>
using namespace std;

  // Assumes  TYPE t; cout << t;  is valid.
template<class TYPE>
inline ostream & operator<< ( ostream & theOstream,
                              const list<TYPE> & theList )
{
  typename list<TYPE>::const_iterator listIterator = theList.begin();
  for ( int i = 0;   listIterator != theList.end();  listIterator ++, i ++ )
    theOstream << "    [" << i << "]:   \"" << (*listIterator) << "\"" << endl;
  return theOstream;
}

struct Functor
{
  bool operator()( const char * a, const char * b )
    { return strcmp(a,b) < 0; }
};

int
main()
{
  list<char*>  l;

    /* Load up some example test data... */
  char  s[3];
  s[2] = '\0';
  for (   s[0]='c'; s[0]>='a'; s[0]-- )
    for ( s[1]='c'; s[1]>='a'; s[1]--  )
      l.push_back(strdup(s));

    /* Show us that test data... */
  cout << l << endl;

    /* Sort list. */
  Functor f;
  l.sort(f);

    /* Show us what we have now... */
  cout << l << endl;
}

Answer 5

As grieve & mrree suggested

simply overloading the () operator works

Thanks everyone who answered

struct Node {
       uint32_t ID;
       char     *       Value;
};

class myClass {
          private:
            uint32_t  myValueLength;
          public:
            list<queueNode *> MyQueue;
            bool operator()(Node * first, Node * second);
            bool doStuff();
}

bool myClass::operator()(Node * first, Node * second) {
    unsigned int ii =0;
    while (ii < myValueLength)
    {
        if (first-> Value[ii] < second-> Value[ii]) 
        {
                return true;
        } else if (first-> Value[ii] > second-> Value[ii])
        {
                return false;
        }

        ++ii;
    }

    return false;
}

bool myClass::doStuff()
{
    list.sort(*this);
}