On Thu, Jan 12, 2012 at 5:23 AM, Dan Ivy <danivy.mail@gmail.com> wrote:
On Thu, Jan 12, 2012 at 8:40 AM, Jeffrey Lee Hellrung, Jr. <jeffrey.hellrung@gmail.com> wrote:
On Wed, Jan 11, 2012 at 9:29 AM, Dan Ivy <danivy.mail@gmail.com> wrote:
Hi,
I was recently trying out Boost.Move and a few issues worth sharing surfaced:
1. It would be helpful to have configuration macros to force emulation mode, even on C++11 compilers, as well as to disable move semantics altogether (that is, the conversion operators to boost::rv& should be disabled, and move/forward should return lvalue-references.
BOOST_RV_REF
and friends should remain intact, so that overloads remain unique.) In many cases, the higher level semantics of a program are expected to be identical under all three configurations, so having a quick way to switch between them is useful during testing/debugging.
It could be useful to force emulation mode, I suppose, but disabling move semantics altogether could easily break code that uses moveable-but-noncopyable types. So I question the utility of the latter, at least.
Forcing emulation mode is useful for people that primarily use compilers where true rvalue-rferences can't be disabled, such as MSVC 10+. It wouldn't set a precedence, anyway. Boost.TypeOf, for example, already has such a macro, for similiar reasons, I suppose. As for disabling move-semantics, I don't feel too strongly about this, and personally never really needed it, yet, but I forsee it as coming in handy at times. You are correct that it will break some code, this is what I was refering to by "in many cases", but move-indifferent code is probably the more common one (after all, rvalue-refs are a relatively recent feature.) I don't think it's too harmful to just have it there, anyway.
I will see what Ion thinks concerning forcing emulation mode. Having thought about it a little more, it could help to test that code developed for C++11 also works in C++03, so I wouldn't be opposed to such a switch. And agreed regarding Boost.TypeOf. Regarding disabling move semantics, again, I fail to see the utility of such a global switch. Such a switch will change the semantics of the code on a global scale. What's wrong with using your own local switch to disable move semantics for just those classes you're interested in?
missing are Boost.Move-aware type traits. Things like add_rvalue_reference are often necessary to calcuate return types of move-aware generic functions, and so on. Whether this belongs in Boost.TypeTraits or Boost.Move is a separate question. Likewise, there should be type
2. Boost.Move is a little bit too opaque, as it stands. What's really traits to
calculate the return types of boost::move and, in particular, boost::forward. On C++11, the return type of forward<T> conincides with add_rvalue_reference<T>, but not so in emulation mode, hence the necessity for this trait.
+1; perhaps Boost.TypeTraits can update the relevant metafunctions to account for boost::rv<T>&? I think we definitely should have
add_reference add_rvalue_reference add_lvalue_reference remove_reference remove_rvalue_reference remove_lvalue_reference is_reference is_rvalue_reference is_lvalue_reference
where boost::rv<T>& and (probably) boost::rv<T> const & are treated as T&&.
One thing I forgot to bring up is how these traits shoud handle types without move emulation. While it would be nice for the result of add_rvalue_reference to be aligned with the return types of boost::move and boost::forward, that is, const T&, for non-emulated types (which also does the reference-collapsing trick), it wouldn't play very nicely with the rest of the traits. (i.e. add_rvalue_ref<T> is const T&, is is_rvalue_ref<const T&> true?) This is another good reason to provide separate traits to calculate the return types of move and forward.
Agreed. So add to the list result_of::move and result_of::forward.
3. For some reason, the emulated boost::move is written so it doesn't accept temporaries. This doesn't play too nicely with forwarding: Unavoidably on C++03, there has to be made a choice whether forwarding functions use non-const or const-qualified references (bad wording, but you get the point), where the former accepts modifiable lvalues and rejects temporaries, and the latter does the opposite (well, sort of). Boost.Move chooses the latter. This means that while BOOST_FWD_REF parameters accept temporaries, these temporaries are bound as lvalues, which pretty much defeats the purpose of the library (note that these temporaries aren't eligible for copy-elision, since they're bound to a reference.)
I wouldn't use such strong language. It's "just" more limited than if true rvalue references were available. You can still pass explicitly created emulated rvalue references, and the library will allow you to move them.
It's not "just" more limited. It's very likely, the way things are now, that someone would pass a "raw" (read un-moved) temporary to a forwarding function, unaware that it wouldn't be moved (because things "just work"). It's even more likely that having been aware of that, he would prefer passing the argument through move(), as ugly as it may be. This is how I see it, anyway. And, as said, boost::move doesn't accept temporaries, so you'll have to make a lvalue out of your temporary, and only then move it. Aside from being even uglier than using move() directly, how would you do that in initializer-lists, for example? (Well, you could use some move_temp, or move(temp()) functions, but, you know, how is this any better?)
All fair criticisms. However, I think this is the simplest solution. If BOOST_FWD_REF( T ) expanded to T&, it wouldn't bind to temporaries, making the call expression ugly, and unnecessarily so in the cases that true rvalue references are available or the type isn't even movable. In any event, the current forwarding solution is no worse than what we had before Boost.Move, so we aren't regressing in any way. If the caller really wanted to ensure their object got moved, there is a way to do that (bind the temporary and explicitly move it, or use a macro like BOOST_MOVE below); if the callee wanted to be particularly helpful, it would offer more overloads to accurately capture moveable temporaries (this can require up to 4 or more overloads and SFINAE, but it's possible). Let's not burden the caller of every call to a BOOST_FWD_REF( T ) parameter. [...]
Ultimately, this is what I suggest:
[...]
Secondly, seriously
consider changing the definition of BOOST_FWD_REF(T) from const T& to T&, as this would acheive two desirable, IMO, goals:: a. Forwarding functions would accept modifiable lvalues (and keep them modifiable). b. Forwarding functions would REJECT temporaries, UNLESS they're passed through boost::move, which assures that they're treated as rvalues.
This makes the use of the interface defined using BOOST_FWD_REF more onerous, *even* if true rvalue references are available! So I would consider this inferior to the current solution, even if the current solution is more limited. And, sometimes, you don't even know if the expression you're using as a parameter to a function of said interface is an rvalue or an lvalue (and sometimes it could be either, if this is all in a template).
Yes it's uglier (but not THAT ugly). Tough luck, portability comes at a price. I'm not sure what you mean by "even if true rvalue references are avialable"? The change I propose only has to do with emulation.
But the call expression doesn't change whether emulation is enabled or not. So you need to uglify the call expression solely to address emulation shortcomings. I don't think that's bad. Better to uglify the interface implementation. [...]
I sympathize with your goal, though. I've actually played around with various modifications of Boost.Move during its development process, and I made some suggestions for additions to Boost.Move which were ultimately rejected to keep the library simple (which is totally understandable). Two such suggestions were:
- An additional macro BOOST_FWD2_REF( T ) which expanded to T& (in emulation mode) and would be used only to capture lvalues or explicitly created (emulated) rvalue references. This included the result of boost::forward.
I though about suggesting something like this, but came to the same conclusion that it overly-complicates stuff. You really don't want to have different types of forwarding functions, and having to know which is which. It's best to have a single one, however unpleasent the consequences are.
In truth, I typically forego BOOST_FWD_REF altogether and just write different overload sets for "outward-looking" interfaces depending on BOOST_NO_RVALUE_REFERENCES. In which case BOOST_FWD2_REF is what I end up using more often.
- An additional macro BOOST_MOVE( expr ) which expanded to an expression
nearly functionally identical to expr except it would explicitly cast the result to an emulated rvalue reference if expr was an rvalue of moveable type.
+1; I see where you're going with this. I think that, given the issues you noted with boost::move taking a reference-to-const, and not knowing the r/l-ness of an expression ahead of time, it's a good solution. Why was it rejected?
I didn't push for it, merely suggested it. Perhaps Ion can address what he thought of the idea. Maybe he didn't see the utility of such a macro. - Jeff