Question

I’m trying to form a std::tr1::unordered_map where the key type is a struct that includes a callback function, for which I’m using std::tr1::function. I’m running into two problems: 1) the function object does not appear to be equality comparable, as the Boost.Function documentation indicates; 2) I don’t see how to implement the hash function, since I can’t get a regular function pointer (or something else I could use for hashing) from the function object.

Here’s the example code:

#include <boost/functional/hash.hpp>
#include <boost/tr1/functional.hpp>
#include <boost/tr1/unordered_map.hpp>
#include <iostream>

int f(int) {}
typedef std::tr1::function<int(int)> callback;

struct Record
{
  callback func;
  // More members...

  // Requirements for unordered_map key.
  friend bool operator==(Record const & lhs, Record const & rhs)
    { return lhs.func == rhs.func; } // error: ambiguous
  friend std::size_t hash_value(Record const & arg)
    { return boost::hash<void *>(arg.func.get()); } // error: no member get()
};

int main()
{
  std::tr1::unordered_map<Record, int> map;
  Record a = {f};
  map[a] = 0;

  return 0;
}

Here’s some detail on the first error :

test.cpp: In function bool operator==(const Record&, const Record&):
test.cpp:16: error: ambiguous overload for operator== in lhs->Record::func == rhs->Record::func
test.cpp:16: note: candidates are: operator==(void (boost::function1<int, int>::dummy::*)(), void (boost::function1<int, int>::dummy::*)()) <built-in>
<root>/boost/function/function_template.hpp:1024: note:                 void boost::operator==(const boost::function1<R, T0>&, const boost::function1<R, T0>&) [with R = int, T0 = int]

For the second error, obviously there’s no function<…>::get member, but what should I use instead?

I’m using Boost version 1.42 and g++ 4.2.2. Thanks for any help.

Update

The answer to the posted question is “you can’t.” tr1::function objects are hashable (e.g., using boost::hash), but are not equality comparable. If you want to use a function in the hash key, rethink the approach or find a workaround.

Answer 1

It would seem that TR1 specifically requires that

template<class Function2> bool operator==(const function<Function2>&);
template<class Function2> bool operator!=(const function<Function2>&);

remain undefined (3.7.2.6), so at the very least you’d have to find another way to get equality. Furthermore, I’m not finding any reference to a get() member method in the paper either.