cpp: make isl objects compatible with standard containers.

[ftynse]

In particular, std::set and std::map expect objects to be less-than-comparable and std::unordered_set and std::unordered_map to have a specialization of std::hash.

C++11 stuff could go in a separate file that is conditionally included based on the C++ version.

[tobiasgrosser]

The bindings are already C++11 of larger. Not sure if this would change anything regarding the separate file.

And yes, I think it would be great to systematically think about less-than and hashability. Your point of needing a std::hash for id was a certainly a good one.

[ftynse]

Sorry, I've mistaken that for the generator file, which is not C++11. Nevermind.

For hashes there are two options: either we can wrap around isl_*_get_hash() or just compute a hash of the pointer with the standard hashing function.

[tobiasgrosser]

This likely depends what you want. isl_*_get_hash gives identical hashes for different objects that model the same set. The pointer does not carry this information.

[ftynse]

Well, I think it is reasonable to expect hashes of logically equal (i.e. represent the same set) values to be equal.

[tobiasgrosser]

There are good reasons for expecting logical equality. On the other side, hashing can be expensive this way. Assume you insert a large isl::set into a hashtable and you first need to scan the full isl::set. I feel there is a tradeoff.