Re: STL hash_map

"Victor Bazarov" <>
Thu, 26 Jul 2007 20:27:58 -0400
Axel Gallus wrote:

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


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

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>;

What is that line supposed to designate? It's not legal to write
a simple type in a statement, without declaring an object or function
with that type.

Besides, you can't pass a non-type as a type template argument.
"Hashy" is an object. The 'hash_map' template expects a *type* as
its third argument.

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

Yes, most likely. Pass it to the _constructor_ when defining your
hash_map<...> _instance_:

    stdext::hash_map<..., Timestamp_Hashing> myMap(Hashy);

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