c++c++11libstdc++libc++

Why does libstdc++ store std::tuple elements in reverse order?


According to Link, with regards to std::tuple...

libstdc++ always places the members in reverse order, and libc++ always places the members in the order given

Assuming that's true, is there a reason (historical or otherwise) why libstdc++ uses reverse order?

Bonus: Has either implementation ever changed its std::tuple ordering for any reason?


Solution

  • See this answer for why libc++ chose forward order. As for why libstdc++ chose reverse order, that is probably because that's how it was demonstrated in the variadics template proposal, and is the more obvious implementation.

    Bonus: No. These orderings have been stable in both libraries.

    Update

    libc++ chose forward storage order because:

    1. It is implementable.
    2. The implementation has good compile-time performance.
    3. It gives clients of libc++ something that is intuitive and controllable, should they care about the order of the storage, and are willing to depend on it while using libc++, despite its being unspecified.

    In short, the implementor of the libc++ tuple merely felt that storing the objects in the order the client (implicitly) specified was the quality thing to do.