[Cython] RFC: an inline_ function that dumps c/c++ code to the code emitter

[Robert Bradshaw]

On Mon, Aug 22, 2016 at 7:44 AM, Kevin Thornton <krthornt at uci.edu> wrote:
> I actually just realized another nagging issue with C++/Cython: member
> typedefs are not supported:
>
> cdef vector[T]:
>       ctypedef size_t size_type
>
> These "scoped" typedefs are very widely-used.  We can live without it, and
> we have been, but while we're making a list.. :)

Yes, that's a good one. Worth an issue :).

> On Sat, Aug 20, 2016 at 11:15 PM Robert Bradshaw <robertwb at gmail.com> wrote:
>>
>> On Fri, Aug 19, 2016 at 9:34 AM, Kevin Thornton <krthornt at 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).
>>
>> > 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 <robertwb at 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.
>> >>
>> >>
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>> >
>> >
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