Feature request: heterogeneous comparison for upsert().

[toojays]

C++14 added heterogeneous comparison lookup to std::map. This allows us to have a map where the key type is std::string, but use a const char * in our lookup, avoiding the construction of a temporary string object.

I'd like for cuckoohash_map::upsert() to support something similar. Let me have a cuckoohash_map with std::string as the key, but pass const char * as the key argument of upsert(), such that a conversion from const char * to std::string is only done if the key needs to be inserted into the table.

Right now for performance reasons I'm using const char * as my key, but it means the caller of my API must ensure the string is valid for the lifetime of the entry in the table, which it doesn't know.

[manugoyal]

I added support for this feature in a new branch 75ad4e0. Since it requires compiling with C++14, while everything else compiles with C++11, I haven't yet bothered to incorporate the unit test into the build system, which I want to do before merging into master. Until then, though, would you mind trying this branch out and seeing if it works for your purposes?

[manugoyal]

Merged into master.

[toojays]

Thanks Manu. It might be a little while before I can test this. :(

Is the only reason you need C++14 because you're using std::equal_to in your test program? I guess I'll find out when I try it, but I'm looking to use this in a C++11 project. Hopefully that's just a matter of defining an appropriate functor, not something I need new language features for.

[manugoyal]

Yeah I'm using std::equal_to, but with a void overload that was added in c++14 and not supported in c++11. The document you linked me explains how that works. Not sure if it's possible to implement such a comparator in c++11 because the void overload is not supported I think.

[toojays]

I'm hoping I can get away with having a non-template function object which just has the necessary overloads of operator()(). If my hashtable has keys of type std::string, as long as my Pred functor has an overload for operator()(const std::string &, const char *) (and maybe vice-versa), it looks like it should work. But I haven't tried it yet.

[manugoyal]

Oh yeah that could work. That's what I'm doing for the hash function in the test, so maybe I can change that for the comparator too.

[manugoyal]

I just pushed an update that removes the c++14 dependency, using an overloaded class instead of std::equal_to<>.

[toojays]

Sorry it took so long for me to test this. It appears to do exactly what I want, and works fine in C++11 mode.

Thanks!