On Fri, Sep 23, 2016 at 09:56:30AM +0000, Edward Welbourne wrote: > Indeed; having all values of some type equivalent is worse (for the "has > my value changed" test you illustrate, at least) than having a value not > equal to itself (which triggers a "changed" event for a non-change). In > fact one can have one's mathematical purity by attacking the "for all x" > part of the above instead of the "x == x" part. Although it's usual to > talk about "a relation is reflexive on S if, for all s in S, it relates > s to s"
Right. That's pretty much the definition the known universe uses: https://en.wikipedia.org/wiki/Reflexive_relation (and no, *I* didn't put it there) > one can equally define reflexive by "whenever the relation > relates any x to y, it also relates x to x and y to y". > (This is the > more natural formulation in a mathematics based on relations [*] rather > than sets.) > > [*] http://www.chaos.org.uk/~eddy/maths/found/relate.xhtml#Types > > With that formulation (which doesn't tie the relation to a set that it's > "on"), returning false for the types that don't support equality > comparison works; we won't consider any value of such a type equal to > *anything*, so our relation isn't required to relate them to themselves. > We are then left with QVariant::operator== being a relation formally on > only those QVariants that support comparison, not on all QVariants > (although it tolerates being asked about the values it doesn't relate to > anything; it just says no about them). How does that help with QHash<QVariant, ...> which needs comparsion of the keys? > It's not ideal (and the justification is heterodox) but false is a valid > answer for the incomparable values, This is a nonsensical line of argument long as you try to use it to counter "unexpected" results. With your "uncomparable things compare false" approach QHash<>::contains() will never return true for uncomparable QVariants, i.e. code like void doSomethingOnlyOncePerValue(QVariant v) { static QHash<QVariant, int> seen; if (seen.contains(v)) return; seen.insert(v, 0); doSomething(v); } will be quite surprising, too. Andre' _______________________________________________ Development mailing list Development@qt-project.org http://lists.qt-project.org/mailman/listinfo/development
