STL hash_map

From:
"Axel Gallus" <uh5d@rz.uni-karlsruhe.de>
Newsgroups:
comp.lang.c++
Date:
Fri, 27 Jul 2007 02:18:21 +0200
Message-ID:
<f8bdks$snp$1@news2.rz.uni-karlsruhe.de>
Hello,

i have a question concerning STL non-standard hash_maps under Visual Studio
2005:

Microsoft STL requires a "hash_compare" object for hash_maps:

template <class Key, class Type, class Traits=hash_compare<Key, less<Key> >,
class Allocator=allocator<pair <const Key, Type> > >
class hash_map

I derived a own class from hash_compare dealing with unsigned integers and
implemented the application operator() and a compare function:

class Timestamp_Hashing : public stdext::hash_compare <unsigned int>

{

public:

/// <summary>

/// Class constructor. The constructor takes two arguments, one defining the
quality of the hash map,

/// the other to adapt to a stream's dts struct.</summary>

Timestamp_Hashing (unsigned int HFactor, unsigned int FTimestamp = 0);

/// <summary>

/// This hash-function is used automatically by "hash_map" for determining
in which bucket to put a certain timestamp.</summary>

/// <param name="Timestamp"> The Timestamp for which the hash-value is to be
caclulated. </param>

/// <return> Returns a hash-value in form of the size_t type which can be
used by iterators.</return>

SIZE_T operator () (const unsigned int & Timestamp) const;

/// <summary>

/// This function is used automatically by "hash_map" for sorting the
timestamps contained in the buckets. </summary>

/// It receives two timestamps and return true if the first one is less the
the second one.

/// <param name="Timestamp1"> First timestamp.</param>

/// <param name="Timestamp1"> Second timestamp.</param>

/// <return> Returns true if Timestamp1 < Timestamp2 </return>

bool operator () (const unsigned int & Timestamp1, const unsigned int &
Timestamp2) const;

private:

// private variables used to save state during hash_map creation

unsigned int Hash_Factor;

unsigned int First_Timestamp;

};

As you can see my derived class Timestamp_Hashing has a constructor taking
two arguments.

I have planned to initiate a certain state of an object of class
Timestamp_Hashing through this constructor which influences the
hash_function.

If i create a hash_map by

stdext::hash_map < unsigned int, unsigned int, Timestamp_Hashing>;

everything compiles well but if i use

Timestamp_Hashing Hashy(10,20);
stdext::hash_map < unsigned int, unsigned int, Hashy>;

the compiler generated an error: Error 11 error C2923: 'stdext::hash_map' :
'Hashy' is not a valid template type argument for parameter '_Tr'.

So, how is it possible to pass a hash_compare/Timestamp_Hashing object with
a certain state to the hash_map constructor - is there a workaround?

Greetings and thx in advance.

Axel

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