Re: Portability of comparing pointers to void

"Matthias Hofmann" <>
Sun, 28 Oct 2007 12:02:47 CST
"Matthias Hofmann" <> schrieb im Newsbeitrag

"Hyman Rosen" <> schrieb im Newsbeitrag

Matthias Hofmann wrote:

Meanwhile I did some further research on partial ordering, strict weak
ordering and total ordering and things are a little clearer for me now, but
there are still two things that keep me wondering:

1.) As far as I understand strict weak ordering, two elements of a set may
be incomparable as long as this incomparability relation is transitive. If
this is true, then the following should be fine?

#include <set>

template <class T> struct strict_weak_less;

template <> struct strict_weak_less<int>
     bool operator()( int a, int b )
         // Define any two elements
         // to be incomparable unless
         // they are equal.
         return a != b;

int main()
     std::set<int, strict_weak_less<int> > intset;

     intset.insert( 2 );
     intset.insert( 4 );

     return 0;

This creates a set where all elements are incomparable with each other. (I
have searched for a real life example for a strict weak ordering that is
a total ordering, but I have not found any. Does anyone know an example?)

2.) Why does the standard require a strict weak ordering for associative
containers, but guarantee a total ordering for std::less<T*> in
20.3.3/8? In
a strict weak ordering, the relation "neither a < b nor b < a" is
transitive, but does not mean equality. Incomparability only means equality
in a total ordering, but doesn't an associative container need to know
whether two elements are equal in order to determine whether the element
already exists? Shouldn't the standard require a total ordering for
std::less for *any* type, not just for pointers?

Matthias Hofmann
Anvil-Soft, CEO - The Creators of Toilet Tycoon - Die Macher des Klomanagers

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