How to pass class public member function as a the template parameter?

Question

I'm working with google sparse hashmap library. And I have the following class template:

template <class Key, class T,
          class HashFcn = std::tr1::hash<Key>,   
          class EqualKey = std::equal_to<Key>,
          class Alloc = libc_allocator_with_realloc<std::pair<const Key, T> > >
class dense_hash_map {
.....
typedef dense_hashtable<std::pair<const Key, T>, Key, HashFcn, SelectKey,
                        SetKey, EqualKey, Alloc> ht;
.....

};

Now I have defined my own class as:

class my_hashmap_key_class {

private:
    unsigned char *pData;
    int data_length;

public:
    // Constructors,Destructor,Getters & Setters

    //equal comparison operator for this class
    bool operator()(const hashmap_key_class &rObj1, const hashmap_key_class &rObj2) const;

    //hashing operator for this class
    size_t operator()(const hashmap_key_class &rObj) const;

};

Now I want to pass my_hashmap_key_class as a Key , my_hashmap_key_class::operator()(const hashmap_key_class &rObj1, const hashmap_key_class &rObj2) as EqualKey and my_hashmap_key_class::operator()(const hashmap_key_class &rObj) as HashFcn to dense_hash_map class as parameters while using it in main function as:

main.cpp:

dense_hash_map<hashmap_key_class, int, ???????,???????> hmap;

What is the proper way of passing the class member functions as template parameters??

I tried passing like:

dense_hash_map<hashmap_key_class, int, hashmap_key_class::operator(const hashmap_key_class &rObj1, const hashmap_key_class &rObj2),hashmap_key_class::operator()(const hashmap_key_class &rObj)> hmap;

But I get compilation error as operator is not detected. Please help me realize where I'm doing wrong.

Answer 1

As discussed in the comments, you should write the equality as an operator== . Also, either make these operators static or remove one of their parameters (the "this" pointer will be the left hand operand for the equality test), otherwise they won't work as expected.

//equal comparison operator for this class
bool operator==(const hashmap_key_class &rObj2) const;

//hashing operator for this class
size_t operator()() const;

Then, your class is ready for client code like this:

my_hashmap_key_class a = ...;
my_hashmap_key_class b = ...;

a == b;   // calls a.operator==(b), i.e. compares a and b for equality
a();      // calls a.operator()(), i.e. computes the hash of a

Then, you should be fine by using the default template parameters.