Re: [review] [sqlite]

From: Klemens Morgenstern (klemensdavidmorgenstern_at_[hidden])
Date: 2024-11-16 13:21:10

On Sat, Nov 16, 2024 at 9:18 PM Ruben Perez <rubenperez038_at_[hidden]> wrote:
>
> vtable.hpp contains the following piece of code:
>
>       auto r = mod.insert(value{argv[1]},
>                           boost::span<value>{reinterpret_cast<value
> *>(argv + 2),
>
> static_cast<std::size_t>(argc - 2)},
>                                              sqlite3_vtab_on_conflict(db));
>
> Where argv is a sqlite3_value **, and sqlite3::value is a wrapper
> class roughly looking like this:
>
> struct value
> {
>     // More member functions
>     explicit value(sqlite3_value * value_) noexcept : value_(value_) {}
> private:
>     sqlite3_value * value_ = nullptr;
> };
>
> I had the impression that such a cast is UB (although it seems to work
> and seems to be accepted by ubsan). Could someone with more knowledge
> about the language point out whether this is allowed or not (and why)
>

I have a static assert in the code, to make sure this works out.

static_assert(sizeof(value) == sizeof(sqlite3_value*), "value must be
same as sqlite3_value* pointer");

If the struct is aligned to the pointer it should be fine. But it
might technically still be UB, I am not sure either.

> Thanks,
> Ruben.