Peter Dimov wrote:
Andrew Sutton wrote:
None of this has anything to do with the fact that you want to require operator== to be an equivalence relation, and it doesn't have to be.
Realistically though, when is == not (at least) an equivalence relation?
Realistically, and in this context, when the two sides are not of the same type.
In most of the cases that I care about, there is some underlying type like "string" or "integer", on which an equivalence relation exists in some formal sense. Then there are some concrete C++ types like std::basic_string<char,allocator_1> or std::basic_string<char,allocator_2> or int32_t or int64_t. operator== is defined on pairs of these concrete types in some way that approximates to the equivalence relation on the underlying formal type, but with some inevitable flakiness at the edges, such as comparison between two char*s or comparison between integers with different numbers of bits. The difficulty, which is perhaps what Dave is grappling with, is how to specify an algorithm that does the "right thing" if-and-only-if the types do the "right thing", while remaining vague about what the "right thing" means to allow the implementation flexibility. Regards, Phil.