On Sat, Aug 20, 2016 at 11:15 PM Robert Bradshaw <rober...@gmail.com> wrote:
> On Fri, Aug 19, 2016 at 9:34 AM, Kevin Thornton <krtho...@uci.edu> wrote: > > Hi Robert, > > > > Related to this point, I think there are three important features that > > Cython would benefit from, as far as C++11 and newer are concerned. In > > order of what I'm guessing to be increased complexity are: > > > > 1. Non-type template parameters. > > This is actually probably the hardest :). There is a PR that's stalled > out, probably needs some attention by both the author and us; it'd be > great to get this in. > > > 2. "rvalue" references for standalone functions and class member > functions: > > > > void foo[T]( T && t ) > > From a callers perspective, one can just declare an extern foo as > foo(T) in Cython, it doesn't care if the actual call is made by value > or by reference--the calling code is exactly the same. > > Actually implementing it for Cython-defined functions (as the callee) > would require additional support, but would probably be pretty easy > (implement a new type, with parsing, which generally behaves the same > as a reference or value from Python's perspective--the actual heavy > lifting happens in the C compiler). > > Want to file an issue on github for this? > > > 3. Variadic templates (member and non-member). More generally, > "parameter > > packs": > > > > http://en.cppreference.com/w/cpp/language/parameter_pack > > This'd be interesting (especially as to how it relates to changing the > arity of functions for concrete instantiations). There's also a > question of syntax. Maybe > > void foo[A, int N, ... Types](A a, Types t): > ... > > It'd likely require #1, e.g. to be able to declare things like > tuple_element::type such that std::tuple.get<N> would be correctly > typed. Unfortunately seems declaring the (somewhat intuitive) API of > std::tuple requires being able to declare a fair amount of its > implementation. > > Still likely feasible though, and I think the notion of variadic > templates is natural enough in a Python setting (which has had > heterogeneously typed tuples since its inception, though of course not > statically typed). > > > #2 is important for efficiency reasons--it isn't just a convenience > feature. > > Without it, Cython cannot support all the member function of std::vector, > > etc. The implication is that some expensive operations, like moving a > big > > object into a vector, require extra copies in Cython while a C++ program > > would just move the object into the new location. > > It's interesting to call this an essential feature, given that C++ > itself didn't have it until recently :). Not that it can't be a big > win, and libraries will more and more probably be designed assuming > it. > > > #3 seems like the hardest, but would enable std::tuple, std::bind, and > other > > handy things to be used in a .pyx file. > > > > Support for "auto" would be nice, but perhaps unrealistic. > > Just leave the cdef out and you basically get auto, i.e. > > my_var = expr > > gives my_var the type of expr. If multiple assignments are made to > my_var, it attempts to find a suitable union type that can hold all > assignments. The one exception is for integers, where it infers the > type to be object iff my_var is used in arithmetic expressions (that > it can't deduce would not overflow with the finite-sized C integer > type). > Wow--that does work (at least for simple cases). It may be nice to have a paragraph about this in the dox? Something like "Emulating auto in C++"? We've been doing a lot of copy/pasting of type names b/c we weren't aware of this. > > > I know that PRs have been submitted, and since stalled out, on a couple > of > > these features. I've tried to look at the Cython parser myself, but I'm > not > > able to follow it, sadly. > > > > Thanks for all your hard work, > > > > Kevin > > > > On Fri, Aug 19, 2016 at 2:39 AM Robert Bradshaw <rober...@gmail.com> > wrote: > >> > >> > >> Though, as you discovered, there are some basic things like non-type > >> template arguments that we would like to have. If there are other > >> specific C++ constructs that are commonly used but impossible to > >> express in Cython it'd be useful to know. > >> > >> > >> _______________________________________________ > >> cython-devel mailing list > >> cython-devel@python.org > >> https://mail.python.org/mailman/listinfo/cython-devel > > > > > > _______________________________________________ > > cython-devel mailing list > > cython-devel@python.org > > https://mail.python.org/mailman/listinfo/cython-devel > > > _______________________________________________ > cython-devel mailing list > cython-devel@python.org > https://mail.python.org/mailman/listinfo/cython-devel >
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