[cfe-dev] Recovering the spelling of a typedef

[Reid Kleckner via cfe-dev]

Can you elaborate on how this typedef information is used for I/O? Do you
mean that it is used by some clang plugin that examines the AST, or
something else?

On Mon, Jul 25, 2016 at 6:55 PM, Keno Fischer via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hi Everyone,
>
> We're trying to integrate the CERN ROOT framework with Julia, both
> of which use LLVM/Clang for C++ interoperability. As such, we're hoping
> to harmonize the versions of clang used in both projects. One major
> obstacle to this currently is a patch that the ROOT folks are carrying
> to support their I/O system which uses the structure of C++ classes to
> determine the on-disk format. The patch as is isn't really in a form
> that could be submitted upstream, but we're hoping to solicit some advice
> to come up with a solution that would be acceptable to clang, and not
> require any local code patches.
>
> With that in mind, let us describe the problem:
>
> As mentioned, ROOT uses the structure of C++ classes to determine it's
> IO format. The one wrinkle to that is that sometimes the I/O storage
> format and the in-memory format are not exactly the same. In particular,
> ROOT has a
>
> typedef double Double32_t;
>
> where if this typedef appears in a struct that is serialized to disk,
> it indicates that it should be stored with 32bit precision on disk, but
> with 64bit precision in memory.
>
> That's *only* for I/O information; for anything regarding symbols we
> need these two to share their instantiation data.
>
> I.e. we want to distinguish the types of D<double>::m and
> D<Double32_t>::m (and also D<vector<Double32_t>>::m and
> D<vector<double>>::m) in
>
> template <class T>
> struct D {
> using type = std::remove_reference<D>;
> T m;
> static int s;
> };
>
> But &D<double>::s must the the same as D<Double32_t>::s; more importantly:
>
> void f(D<double>);
>
> must be called by f(D<Double32_t>{}). That is (IIRC) in contrast of what
> the C++ committee discussed for strong typedefs.
>
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>
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