From:

Ulrich Eckhardt <eckhardt@satorlaser.com>

Newsgroups:

microsoft.public.vc.language

Date:

Tue, 19 May 2009 12:37:00 +0200

Message-ID:

<cneae6-vd6.ln1@satorlaser.homedns.org>

I try to implement a hash_set (hash_map - both) for a class which has no

ordering relation (operator < etc), but just has an operator== and !=.

in earlier STL versions the hash_set/map template has arguments for a

hash class and for an equal_to class.

ordering relation (operator < etc), but just has an operator== and !=.

in earlier STL versions the hash_set/map template has arguments for a

hash class and for an equal_to class.

FYI: You mean the C++ standard library, not the STL.

STL in VC9 does not have these arguments, but just has an hash_compare

which expects an instance of class less<Type>.

I do not expect an hash collection to sort the elements, it should just

store the elements based on an hash value and allow fast access.

which expects an instance of class less<Type>.

I do not expect an hash collection to sort the elements, it should just

store the elements based on an hash value and allow fast access.

If two values have the same hash, the container must still be able to tell

them apart. The hash containers primarily work with a hash to speed up

things but at the second level use normal comparisons to provide guarantees

that are not given by hashes.

i read the documentation and the docu says:

================================

In general, the elements need be merely less than comparable to establish

this order: so that, given any two elements, it may be determined either

that they are equivalent (in the sense that neither is less than the

other) or that one is less than the other. This results in an ordering

between the non-equivalent elements. On a more technical note, the

comparison function is a binary predicate that induces a strict weak

ordering in the standard mathematical sense. A binary predicate f(x,y) is

a function object that has two argument objects x and y and a return value

of true or false. An ordering imposed on a hash_set is a strict weak

ordering if the binary predicate is irreflexive, antisymmetric, and

transitive and if equivalence is transitive, where two objects x and y are

defined to be equivalent when both f(x,y) and f(y,x) are false. If the

stronger condition of equality between keys replaces that of equivalence,

then the ordering becomes total (in the sense that all the elements are

ordered with respect to each other) and the keys matched will be

indiscernible from each other. =================================

Do i understand the documentation correctly in the sense, that it is ok to

implement template less<T> simple by returning a!=b ?

================================

In general, the elements need be merely less than comparable to establish

this order: so that, given any two elements, it may be determined either

that they are equivalent (in the sense that neither is less than the

other) or that one is less than the other. This results in an ordering

between the non-equivalent elements. On a more technical note, the

comparison function is a binary predicate that induces a strict weak

ordering in the standard mathematical sense. A binary predicate f(x,y) is

a function object that has two argument objects x and y and a return value

of true or false. An ordering imposed on a hash_set is a strict weak

ordering if the binary predicate is irreflexive, antisymmetric, and

transitive and if equivalence is transitive, where two objects x and y are

defined to be equivalent when both f(x,y) and f(y,x) are false. If the

stronger condition of equality between keys replaces that of equivalence,

then the ordering becomes total (in the sense that all the elements are

ordered with respect to each other) and the keys matched will be

indiscernible from each other. =================================

Do i understand the documentation correctly in the sense, that it is ok to

implement template less<T> simple by returning a!=b ?

No. You can not derive an ordering from the existing comparison operators.

You could do the reverse though, because '!(a<b) and !(b<a)'

implies 'a==b'.

class my_hash_compare

: public hash_compare<A,less<A>>

{

......

bool operator()(const A& _Keyval1, const A& _Keyval2) const

{

// ?????????

return _Keyval1 != _Keyval2;

}

Is this correct?

: public hash_compare<A,less<A>>

{

......

bool operator()(const A& _Keyval1, const A& _Keyval2) const

{

// ?????????

return _Keyval1 != _Keyval2;

}

Is this correct?

No, see above. In addition you are not allowed to use names like _Keyval,

those are reserved.

what happens with this less<A> which i pass as argument? i do not have an

operator < for objects of type A so how is this less<A> be instantiated?

operator < for objects of type A so how is this less<A> be instantiated?

std::less<T> per default uses operator<. You can therefore simply provide

operator< to make std::less work as is. Alternatively, if T is not a type

where a less-than comparison makes sense, you can provide a functor like

std::less, though the hash_compare interface requires some additional

things, if I understand correctly.

Suggestion: as a first step, use std::set or std::map instead of hash_set or

hash_map. If you manage to provide a suitable comparator to those (and it

is easier to locate good examples on the web), adapting them to the hash

containers will be a breeze.

class my_hash_compare

: public hash_compare<A,less<A>>

{

public:

size_t operator()(const A & _Keyval) const

{

int v = _Keyval.val;

return hash_value(v);

}

bool operator()(const A& _Keyval1, const A& _Keyval2) const

{

// ?????????

return _Keyval1 != _Keyval2;

: public hash_compare<A,less<A>>

{

public:

size_t operator()(const A & _Keyval) const

{

int v = _Keyval.val;

return hash_value(v);

}

bool operator()(const A& _Keyval1, const A& _Keyval2) const

{

// ?????????

return _Keyval1 != _Keyval2;

return _Keyval1.val < _Keyval2.val;

}

};

};

Uli

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