Olivier Goffart > What use case did you have in mind where a reflexive relation is any usefull?
Well, having x == x for all x is generally considered a useful property of equality. That's all "reflexive" is saying. > There is the case of the key of a QHash or in a QSet, but even then i'm not > sure it is wise that all QVariant of the same type map to the "same" value. 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" 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). It's not ideal (and the justification is heterodox) but false is a valid answer for the incomparable values, Eddy. _______________________________________________ Development mailing list Development@qt-project.org http://lists.qt-project.org/mailman/listinfo/development
