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Re: [std-proposals] Relocation in C++

From: Sébastien Bini <sebastien.bini_at_[hidden]>
Date: Tue, 20 Sep 2022 10:54:30 +0200
Hi all,

There's a problem here; you can't pass arrays by value. An array-typed
> function parameter is decayed to a pointer in the signature, so `arr` is
> actually of type `T*`. I'm not sure what can be done here; perhaps expand
> out arrays into their elements?

Indeed... I am not in favor of expanding arrays, as the other subobjects
are not automatically expanded. Maybe pass them as std::array? Not a fan of
this either... Your std::decompose is more promising.

> When structured relocation is used we can issue a warning if a
>> get_bindings is declared for the class type but its parameters don't match
>> (it may very well lead to an error if other binding protocols fail to
>> apply). That way we can warn the user that they forgot to update the
>> get_binding function when they changed their class layout.
>> I know it's unorthodox, but it's safe. I wonder how it mixes with virtual
>> inheritance; maybe we should forbid such types...
> Yes, virtual bases would have to be banned - or, at least, complete types
> with overlapping immediate subobjects.


I almost want to propose a magic library function that takes a prvalue, a
> list of immediate bases and pointers to data members, and returns those
> subobjects as a destructurable class type. Something like:
> auto [b, x, arr] = std::decompose<B, &D::x_, &D::arr_>(reloc obj);
> It would check that the returned bases and data members are direct,
> distinct, non-overlapping and relocatable, and destroy any subobjects not
> mentioned. Then you would be able to return that `std::decompose` result
> directly, or perform further processing it.

This looks good, but how are you able to mix type (B) and non-type (&D::x)
parameters in the function declaration?

If we have such a function, then we can mix it with a get/get_bindings API,
keeping the recursive bit:

auto get_bindings(E e)
    auto [base, x, arr] = std::decompose<B, &D::_x, &D::_arr>(reloc e);
    auto [a1, a2] = reloc arr;
    return std::tuple(reloc base, reloc x, reloc a2);

That also allows passing unaccessible (private) members or bases to

It would also pose problems for immovable types that must be constructed
> in-place by conversion or converting constructor... although those are
> probably rather more rare than your scenario.
>> I can think of another solution:
>> template <class... UTypes>
>> std::tuple::tuple(UTypes... types)
>> Using this constructor, all parameters must be passed as prvalues. This
>> means for users to use reloc on a parameter they wish to relocate,
>> std::cref on a parameter they wish to copy, and a new std::mref on a
>> parameter they wish to move (std::mref being a helper function that builds
>> a new std::move_reference_wrapper. We cannot use std::move because it
>> returns an xvalue, not a prvalue...). Thankfully for us, std::cref and
>> std::mref return a prvalue. This tuple constructor would be able to detect
>> the two kinds of reference wrappers and unwrap them to call the appropriate
>> constructor.
>> However I see two potential problems. First, I am not sure how things
>> might conflict against the tuple(UTypes&&... types) constructor. Second
>> users have been taught to use std::move since C++11, and now they would
>> need to use std::mref in some circumstances...
> Yes, I think if this constructor were to be provided it would need to take
> a tag argument as first parameter, for disambiguation.


> Another workaround would be to add a relocating_wrapper<T> that wraps a
> std::optional<T> and has a one-shot conversion operator to T:
> T relocating_wrapper<T>::operator T() && { return opt_.pop(); }
> Users would then write std::tuple<std::string, RelocOnlyTp, int>(reloc a,
> std::relocating_wrapper(reloc b), reloc c) and it would work... sort of.

That could also do. But here we introduce a reloc_wrapper which leaves an
open door for bugs (operator T() called twice). It also has the cost of a
double relocation (once inside the wrapper, once out).
On the other hand, with: `template <class... UTypes> tuple(relocate_tag,
UTypes... types)`: nothing is relocated twice. We introduce a wrapper as
well (mref), but it's a safe one. And even if std::move is used by mistake
instead of std::mref then thanks to the tag the right constructor is still
picked, and a temporary is created (suboptimized but harmless).

A rather more ambitious possibility would be to invent a new type of
> deduction for perfect forwarding:
> std::tuple::tuple(decltype(auto)... args); // args deduced to
> decltype of arguments so T, T&, T&& dependent on value category
> This currently parses and is rejected at the semantic level, so it's "free
> real estate", so to speak. There may be other available syntaxes, of course.

This would be a solution, but then how do we forward the parameters? We
don't have a `reloc_of_forward` function. Even if we manage to make one
(new reloc... operator?), then we will end up with the prvalue parameters
being destructively relocated, while the others just got their reference
passed along. This makes the prvalues no longer usable (as somehow they are
passed to reloc), which in turn should make the whole argument pack

I believe there are things to be done in that area, but maybe for a second
proposal? This one is already large enough, and there is no strong need to
do that now (AFAICT), as we can get around with std::mref or

Received on 2022-09-20 08:54:43