Polymorphic types aren’t trivially relocatable

One of the (minor, non-key) differences between P1144 trivial relocatability and P2786R1 trivial relocatability (§11.3, page 19) is that P2786R1 accidentally considers polymorphic types to be trivially relocatable by default. I say “accidentally” because of course this cannot really be permitted. Godbolt:

struct B {
    virtual int getter() const { return 1; }
    virtual ~B() = default;
struct D : B {
    int d = 2;
    int getter() const override { return d; }

alignas(D) char dbuf[sizeof(D)];
alignas(B) char bbuf[sizeof(B)];
B *d = ::new (dbuf) D();
B *b = (B*)bbuf;
printf("Expect 1: %d\n", b->getter());

The “RELOCATE” comment should be replaced with the code to relocate a B object from *d into *b. In P1144, the spelling is simply

std::relocate_at(d, b);

and the library decides whether to use trivial relocation behind the scenes. In P2786R1, the spelling is

std::trivially_relocate(d, d+1, b);

In P1144, B is not considered trivially relocatable, and the code above Just Works.

Why is B not trivially relocatable? P1144 offers two answers: One, at the compiler level, B doesn’t meet [basic.types.general]’s criterion that a trivially relocatable type mustn’t have virtual member functions. Two, at the human level, we can see that B fails to model [concept.relocatable]’s semantic requirements.

In P2786R1, B is (“accidentally”) considered trivially relocatable, and the code above has undefined behavior at runtime.

*d’s vptr is memmoved into *b, and then the virtual call b->getter() invokes D::getter through that (now-erroneous) vptr, which tries to access the data member d of a D object that doesn’t actually exist.

The Godbolt above uses my own Clang/libc++ implementation of P1144R8, and Corentin Jabot’s Clang implementation of P2786R0 (with none of the library components). I’ve told Corentin he should feel free to adopt any pieces of my libc++ implementation that would help get the P2786 demonstration into a working state. Of course, the authors of P2786 could also help by refactoring P2786’s library clauses to match P1144’s. :) As you can see, the P1144 library facilities tend to be more usable than P2786’s.

This might be a good time to remind folks that my Clang/libc++ implementation also provides a [[clang::maybe_trivially_relocatable]] attribute implementing P2786’s “dull knife” semantics (but without P2786R1’s sharp-when-you-didn’t-want-it parts, like its treatment of polymorphic types). So you can experiment with a full libc++ implementation of dull-knife semantics simply by passing -D_LIBCPP_TRIVIALLY_RELOCATABLE_IF(x)=[[clang::maybe_trivially_relocatable(x)]]. (Godbolt.)

Posted 2023-06-24