dblaikie added inline comments.
================
Comment at: clang/bindings/python/clang/cindex.py:1530
+
+ def record_needs_implicit_default_constructor(self):
+ """Returns True if the cursor refers to a C++ record declaration
----------------
aaron.ballman wrote:
> royjacobson wrote:
> > anderslanglands wrote:
> > > royjacobson wrote:
> > > > anderslanglands wrote:
> > > > > royjacobson wrote:
> > > > > > aaron.ballman wrote:
> > > > > > > aaron.ballman wrote:
> > > > > > > > I don't think we should expose any of the "needs" functions
> > > > > > > > like this -- those are internal implementation details of the
> > > > > > > > class and I don't think we want to calcify that into something
> > > > > > > > we have to support forever. As we add members to a class, we
> > > > > > > > recalculate whether the added member causes us to delete
> > > > > > > > defaulted special members (among other things), and the "needs"
> > > > > > > > functions are basically used when the class is completed to
> > > > > > > > handle lazily created special members. I'm pretty sure that
> > > > > > > > lazy creation is not mandated by the standard, which is why I
> > > > > > > > think the "needs" functions are more of an implementation
> > > > > > > > detail.
> > > > > > > CC @erichkeane and @royjacobson as folks who have been in this
> > > > > > > same area of the compiler to see if they agree or disagree with
> > > > > > > my assessment there.
> > > > > > I think so. The 'needs_*' functions query `DeclaredSpecialMembers`
> > > > > > and I'm pretty sure it's modified when we add the implicit
> > > > > > definitions in the class completion code. So this looks a bit
> > > > > > suspicious. Is this API //meant// to be used with incomplete
> > > > > > classes?
> > > > > > For complete classes I think looking up the default/move/copy
> > > > > > constructor and calling `isImplicit()` is the way to do it.
> > > > > >
> > > > > > About the 'is deleted' API - can't the same be done for those
> > > > > > functions as well so we have a smaller API?
> > > > > >
> > > > > > If this //is// meant to be used with incomplete classes for
> > > > > > efficiency that would be another thing, I guess.
> > > > > >
> > > > > So the intended use case here is I'm using libclang to parse an
> > > > > existing C++ libray's headers and generate a C interface to it. To do
> > > > > that I need to know if I need to generate default constructors etc,
> > > > > which the needs* methods do for me (I believe). The alternative is I
> > > > > have to check manually whether all the constructors/assignment
> > > > > operators exist, then implement the implicit declaration rules myself
> > > > > correctly for each version of the standard, which I'd rather avoid.
> > > > >
> > > > > Would putting a note in the doc comment about the behaviour differing
> > > > > when the class is being constructed as originally suggested work for
> > > > > everyone?
> > > > Why is the `__is_default_constructible` builtin type trait not enough?
> > > > Do you have different behavior for user provided and implicit default
> > > > constructors?
> > > >
> > > Can I evaluate that from libclang somewhow? I can't modify the C++
> > > libraries I'm wrapping.
> > >
> > > Basically, given:
> > > ```
> > > struct Foo { /* ... */ };
> > > ```
> > >
> > > I want to generate:
> > >
> > > ```
> > > typedef struct Foo_t;
> > >
> > > Foo_t* Foo_ctor();
> > > Foo_t* Foo_copy_ctor(Foo_t*);
> > > /* etc... */
> > > Foo_dtor(Foo_t*);
> > > ```
> > >
> > > In order to know which ones to generate for an arbitrary struct that may
> > > or may not have any combination of ctor/assignments defined, I need to
> > > know which ones exist and follow the implicit generation rules for the
> > > ones that don't. I can do this myself with a whole bunch of
> > > version-dependent logic, but I'd rather just rely on libclang since it
> > > already knows all this much better than I do.
> > I looked a bit, and it seems they aren't, and that generally libclang
> > doesn't really know about Sema, so exporting the type traits is not that
> > easy :/
> >
> > I'm not sure what's the best way forward here, but I don't like the idea of
> > exporting those half baked internal API calls when there are actual
> > standardized and implemented type traits that perform the same goal.
> CCing folks who may have more historical memory of the C APIs and whether
> they're expected to operate on a completed AST or are expected to work on an
> AST as it is under construction. My unverified belief is that these APIs are
> expected to work on a completed AST.
>
> @echristo @dblaikie @rjmccall @rsmith
>
> I'm also not certain of what the best path forward is here. I'm not
> comfortable exposing the needs* functions because they really are
> implementation details and I don't want to promise we'll support that API
> forever. But at the same time, the use case is reasonably compelling on the
> assumption you need to inspect the AST nodes as they're still under
> construction instead of inspecting them once the AST is completed. If the AST
> is fully constructed, then we should have already added the AST nodes for any
> special member functions that needed to be generated implicitly, so as Roy
> mentioned, you should be able to find the special member function you're
> after and check `isImplicit()` on it.
Not sure I'm quite following - it doesn't look (admittedly, sorry, at a
somewhat superficial look at the discussion here) like this is necessarily
about incomplete AST - could parse the header and stop. That's a complete AST,
yeah? And then it might be OK/reasonable to ask "could this type be default
constructed" (even if the implicit ctor has been implicitly instantiated/there
was no use in the source code that's been parsed)
Is that correct?
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D135557/new/
https://reviews.llvm.org/D135557
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