如何仅专门化具有多个参数类型的模板类中的一种方法

Question

I am implementing a HashTable template class in C++ and its prototype is like this:

template<class K, class T, unsigned int containerSize=CONTAINER_SIZE>
class LPHashTableChained{
    ........

    unsigned int hashFunction(K& key);

}

My question is how I can specialize my hashFunction() method to behave differently when K equals string type.

I tried to implement the function with its correct format and with a second implementation where I omit the class K argument and put string as the type like below:

1st implementation:

template<class K, class T, unsigned int containerSize>
unsigned int LPHashTableChained<K,T,containerSize>::hashFunction(K& key){

}

2nd implementation:

template<class T, unsigned int containerSize>
unsigned int LPHashTableChained<string,T,containerSize>::hashFunction(const string& key){

}

But I get compilation errors !!!

What is the easiest way to specilize this hashFunction when K= string ???

Thanks

Answer 1

You cannot partially specialize member functions of templates. (Total specializations are fine, though.)

The best approach for your class is to do as the standard library does, though, and provide the hash function as a "policy"-type template argument:

template <typename K, typename V, typename Hash = std::hash<K>>
class HashTable
{
  Hash hasher;
  // use hasher(x)
};

Now you can simply specialise the hasher for your string type, or provide your own:

// provide custom

struct MyInsaneHasher { std::size_t operator()(const Foo &) const; };
HashTable<Foo, int, MyInsaneHasher> m1;

// add specialization for `std::hash`:

namespace std
{
  template <> struct hash<Bar> { size_t operator()(const Bar&) const; };
}
HashTable<Bar, int> m2;