C++ comparison with LEDA library

Question

I am trying to create a Set using the LEDA library... I am adding elements of a Class which has a compare() method defined under namespace LEDA... Unfortunately, the compiler is unable to locate the compare function for this class... Here's the error message...

/home/user/Desktop/leda/incl/LEDA/core/set.h: 
In constructor ‘leda::set<E, set_impl>::set() [with E = Operator*, set_impl = leda::avl_tree]’: 
../src/suite.cc:52:   instantiated from here /home/user/Desktop/leda/incl/LEDA/core/set.h:71: error: no matches converting function ‘compare’ to type ‘int (*)(class Operator* const&, class Operator* const&)’ 
    /home/user/Desktop/leda/incl/LEDA/internal/param_types.h:351: error: candidates are: int leda::compare(const char&, const char&) 
    /home/user/Desktop/leda/incl/LEDA/internal/param_types.h:352: error:                 int leda::compare(const unsigned char&, const unsigned char&) 
    /home/user/Desktop/leda/incl/LEDA/internal/param_types.h:353: error:                 int leda::compare(const int&, const int&) 
    /home/user/Desktop/leda/incl/LEDA/internal/param_types.h:354: error:                 int leda::compare(const unsigned int&, const unsigned int&) 
    /home/user/Desktop/leda/incl/LEDA/internal/param_types.h:355: error:                 int leda::compare(const long int&, const long int&) 
    /home/user/Desktop/leda/incl/LEDA/internal/param_types.h:356: error:                 int leda::compare(const long unsigned int&, const long unsigned int&) 
    /home/user/Desktop/leda/incl/LEDA/internal/param_types.h:357: error:                 int leda::compare(const float&, const float&) 
    /home/user/Desktop/leda/incl/LEDA/internal/param_types.h:358: error:                 int leda::compare(const double&, const double&) 
    /home/user/Desktop/leda/incl/LEDA/core/string.h:382: error:                 int leda::compare(const leda::string&, const leda::string&)

LEDA requires the compare() method be defined for elements of a set.

Suite::Suite (set<Operator*> *ops)
  : operators(ops!=NULL ? ops : new set<Operator*>)
{

I have this compare method defined according to LEDA requirements...

namespace leda {
inline int compare (Operator* const &a, Operator* const &b)
{
  return a==b ? 0 : a<b ? -1 : 1;
}
};

But it still cannot find it in LEDA librabry's set.h here..

 set() { cmp_ptr = compare; }

It tries to find the pointer to the compare() method and assigns it to cmp_ptr ... but cannot find it...

I've defined the method but somehow it doesn't get recognized?

Update: I seem to be having this problem with all compare() definitions.... including other classes in Sets..

Answer 1

Ok, the bug was weird! The compare() method must be defined prior to including the LEDA headers....

Answer 2

Attaching a much simpler example that reproduces the same error... You will need to download the free version of the LEDA library for this.. but it is painless to use... http://www.algorithmic-solutions.com/leda/ledak/index.htm

This is the header file... pair.h

#include <istream>
#include <LEDA/core/set.h>

using leda::set;

class pair {
private:
      int  x;
      int  y;

public:
    pair() { x = y = 0; }
    pair(const pair& p) { x = p.x; y = p.y; }
    pair& operator=(const pair& p) {
        if(this != &p) { x = p.x; y = p.y; }
        return *this;
    }

    friend std::istream& operator>> (std::istream& is, pair& p) 
    { is >> p.x >> p.y; return is; }

    friend std::ostream& operator<< (std::ostream& os, const pair& p) 
    { os << p.x << " " << p.y; return os; }

    pair(int a, int b) { x = a;  y = b; }

    int get_x() const { return x; }
    int get_y() const { return y; }
};

namespace leda {
    inline int compare(const pair& p, const pair& q)
    {
        if (p.get_x() < q.get_x()) return  -1;
        if (p.get_x() > q.get_x()) return   1; 
        if (p.get_y() < q.get_y()) return  -1;
        if (p.get_y() > q.get_y()) return   1;
        return 0;
    }
};

This is the main c++ file pair.cc

#include<pair.h>

using std::cout;
using std::endl;

main() {

    set<pair> T;
    cout<<"Hello world"<<endl;
}

Simply compile them using the following command:

g++ -I$LEDA_INCLUDE_PATH -I$PATH_TO_PAIR.H pair.cc