Re: Yet another template class question

"Alf P. Steinbach" <>
Fri, 31 Aug 2007 17:32:14 +0200
* 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


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.


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

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


   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

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A: Top-posting.
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