Yet another template class question

[Anonymous]

I am writing a template HashTable class. I have got it working, but I
want to make the code more generic and more robust. The current code
looks something like this:

template <class InnerType, class KeyType, class KeyToSizeT>
class myHash
{
// Impl here
};

Where:

InnerType is the data type being stored
KeyType is the data type of the hash key
KeyToSize is the function that returns the size of the hash key


InnerType now MUST now implement interface IHashable:


class myHashKey
{
virtual bool operator==(const myHashKey& key) const = 0 ;
virtual size_t Size() const = 0 ;
};


class IHashable
{
virtual const myHashKey& GetKey() const = 0 ;
};




With this information, I now want to be able to simplify my MashTable
class to:


template <class InnerType>
class myHash
{
//Ho do I reference KeyType and KeyToSizeT now ?
// Impl here
};

 

[Alf P. Steinbach]

* Anonymous:

I am writing a template HashTable class. I have got it working, but I
want to make the code more generic and more robust. The current code
looks something like this:

template <class InnerType, class KeyType, class KeyToSizeT>
class myHash
{
// Impl here
};

Where:

InnerType is the data type being stored
KeyType is the data type of the hash key
KeyToSize is the function that returns the size of the hash key


InnerType now MUST now implement interface IHashable:


class myHashKey
{
virtual bool operator==(const myHashKey& key) const = 0 ;
virtual size_t Size() const = 0 ;
};


class IHashable
{
virtual const myHashKey& GetKey() const = 0 ;
};

This sounds more like a "concept" (some GetKey function must be present,
result must support equality checking and Size) than an OO interface.

With this information, I now want to be able to simplify my MashTable
class to:


template <class InnerType>
class myHash
{
//Ho do I reference KeyType and KeyToSizeT now ?
// Impl here
};

You can always associate a type with other types and functions via
template specializations.

E.g.

template< typename T > struct HashKeyFor{ typedef T Type; };

template<> struct HashKeyFor<double>{ typedef FooType Type; };
template<> struct HashKeyFor<str::string>{ typedef BarType Type; };

then

template< class InnerType >
class MyHash
{
typedef HashKeyFor<T>::Type HashKey;
// ...
};

You might want to google for "traits", which is just about the same
idea, only with a traits class several such associated types etc. are
usually collected in the same class.

Cheers, & hth.,

- Alf