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Subject: Re: [std-proposals] The C++ 17&&20 object model is broken, and concepts would reveal it for the world to see
From: Arthur O'Dwyer (arthur.j.odwyer_at_[hidden])
Date: 2019-10-04 19:20:08


On Fri, Oct 4, 2019 at 7:50 PM Omer Rosler via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> This is meant to be a discussion before I write a formal proposal.
> My presentation is not very clean, so please forgive me.
> I wrote this in a proposal style already, but it is not final in any way.
> Please tell me what do you think.
> Is my estimation of the importance of this problem realistic?
> Is my solution tractable?
> Does it break any C++ 14 code not already broken in 17/20?
>

Can you post sample code illustrating the problem?
The code you posted does not compile. I count several problems.
- Fundamentally, the line `a += b += b` is NOT equivalent to `a += 2*b`,
because the former modifies `b` and the latter doesn't. I think you meant
`a += b + b;`
- Your `lazy_addable_wrapper & operator+=(B &&a)` takes a forwarding
reference, but unconditionally moves from it; you should forward from it
instead.
- The line `std::move(std::back_inserter(cached_additions),
other.cached_additions.begin(), other.cached_additions.end());` certainly
will not compile. You got the arguments in the wrong order.
- The line `wrapper+=3+= lazy_addable_wrapper( lazy_addable_wrapper(arr[7])
+= (-5));` shouldn't compile, because you can't `+=` an rvalue such as `3`.
Again, I think you meant `wrapper += 3 + law(law(arr[7]) + -5);`.
- Constraints of the form `requires X<T> && !Y<T>` probably do not behave
the way you expect them to. Negated constraints are always a bad idea.
- You never define `CrossTypeAddable`.
- You never define `Addable` either.
- You never define `Copyable` or `Movable` either, but at least I'm a
pretty confident guesser in those cases.

Here's your code, cut-and-pasted and very mildly cleaned up, but it still
doesn't illustrate whatever issue you're talking about.
https://concepts.godbolt.org/z/CBS90R
Can you alter it so that it does show the issue you care about?

-Arthur

>
> *The problem*
>
> Writing a generic library that need to create objects of the types it
> uses, is impossible to implement correctly in C++ 20 without havy
> type-based-metaprogramming.
>
> Why? because concepts know nothing of storage, and by extension, on the
> correct value semantics usage of these objects (should I copy, move,
> forward, allocate them on the heap?).
> This means that in-order to extract the value semantics (and dispatch
> accordingly) requires heavy SFINAE magic.
>
> This exposes a hole in the object model introduced in C++ 17.
>
> We'll consider a single toy example that illustrates all of the problems
> this causes (by the way, the `std::error` type proposed as part of the
> "Herbceptions" proosal suffers from the same problem).
>
> If you can write the example below correctly, then you must understand the
> problem.
>
> If you want to see the core issue, skip the following sub-section.
> If you want to understand why it will be an issue of major importance
> after concepts ship, read the example code first.
>
> Toy example: lazy_addable_wrapper library
>
> Look at a toy example of a concepts-based generic library that caches `+=`
> operations on a value and applies all when requested.
>
> Consider this library as abuilding block of two other generic libraries:
> 1. A "safe array smart-pointer" library that does not enable out of bounds
> access by remembering the original arrays used to create the pointers.
> 2. An expression template library that can convert `a += b += b` into
> `a += (2*b)` at compile time (consider the type used to be inefficient to
> move, such as matrices).
>
> ```c++
> template <Addable A>
> struct lazy_addable_wrapper
> {
> //special member functions skipped for breviety
> template <typename B = A>
> requires std::is_convertible_v<B, A>
> constexpr lazy_addable_wrapper(B &&b) : cached_additions{},
> starting_value(std::forward<B>(b))
> {
> }
> template <Movable B = A>
> requires Addable<B>
> constexpr lazy_addable_wrapper & operator+=(B &&a)
> {
> cached_additions.emplace_back(std::move(a));
> return *this;
>
> }
> // we still need forwarding reference because we want to allow
> prvalue references to non-movable types to choose this overload
> template<Copyable B = A> requires Addable<B> && !requires Movable<B>
> constexpr lazy_addable_wrapper& operator+=(B&& a) {
> cached_additions.push_back(std::forward(a)); return *this; }
> template <Movable B = A>
> requires Addable<B> constexpr lazy_addable_wrapper
> &operator+=(lazy_addable_wrapper<B> &&other)
> {
> (*this)+=
> std::forward<decltype(std::forward<lazy_addable_wrapper<B>>(other).starting_value)>(std::forward<lazy_addable_wrapper<B>>(other).starting_value);
> std::move(std::back_inserter(cached_additions),
> std::make_move_iterator(other.cached_additions.begin()),
> std::make_move_iterator(other.cached_additions.end()));
> return *this;
>
> }
> template <Movable B = A>
> requires Addable<B> && !Copyable<B>
> constexpr lazy_addable_wrapper & operator+=(const
> lazy_addable_wrapper<B> &other) = delete;
> template <Movable B = A>
> requires Addable<B> && Copyable<B>
> constexpr lazy_addable_wrapper &operator+=(lazy_addable_wrapper<B>
> &&other)
> {
> (*this)+=
> std::forward<decltype(std::forward<lazy_addable_wrapper<B>>(other).starting_value)>(std::forward<lazy_addable_wrapper<B>>(other).starting_value);
> std::move(std::back_inserter(cached_additions),
> std::make_move_iterator(other.cached_additions.begin()),
> std::make_move_iterator(other.cached_additions.end()));
> return *this;
>
> }
> template <Copyable B = A>
> requires Addable<B> && !Movable<B>
> constexpr lazy_addable_wrapper & operator+=(const
> lazy_addable_wrapper<B> &other)
> {
> (*this)+=
> std::forward<decltype(std::forward<lazy_addable_wrapper<B>>(other).starting_value)>(std::forward<lazy_addable_wrapper<B>>(other).starting_value);
> std::move(std::back_inserter(cached_additions),
> other.cached_additions.begin(), other.cached_additions.end());
> return *this;
>
> }
> constexpr A&& operator*() &&
> {
> return std::accumulate(cached_additions.begin(),
> cached_additions.end(), std::move(starting_value), [](auto &&lhs, auto
> &&rhs) { return std::forward<decltype(lhs)>+=
> std::forward<decltype(rhs)>(rhs); });
>
> }
>
> private:
> std::vector<A> cached_additions;
> A starting_value;
> };
> //deduction guides skipped for breviety
>
> static_assert(CrossTypeAddable<lazy_addable_wrapper<int[10]>, ptrdiff_t>,
> "What?");
> static_assert(CrossTypeAddable<
> lazy_addable_wrapper<
> lazy_addable_wrapper<int[10]>>,
> lazy_addable_wrapper<ptrdiff_t>>, "Wierd, and problematic for
> multi-dimensional arrays");
> ```
>
> I counted at least 4 bugs that cause these static assertions to fire
> (there are probably more).
>
> Two of them are fixable "inline".
> One requires to make specializations to the entire templated class.
> Can you list all of them?
>
> The second static assertion would fire, and it is inheritly unfixable in
> C++ 20.
>
> All of these bugs come from the case of non-copyable non-movable types.
> They still bind to the references (as they are prvalues), but then, the
> function body fails to compile.
>
> The core of the problem
> One of the most fundamental invariants of C++'s value semantics:
>
> *User defined types are equivalent to builtin types*
>
> This is true when we consider glvalues but not for *prvalues*.
>
> With C++ 17's guaranteed copy elision, elision became part of the object
> model, but users can't possibly write code that depends on it.
> For example:
>
> ```c++
> int arr[10][12];
> lazy_addable_wrapper arr_ptr(arr[3]);
> wrapper+=3+= lazy_addable_wrapper( lazy_addable_wrapper(arr[7]) += (-5));
> ```
>
> Where all required constructors, conversions functions and deduction
> guides are implemented correctly - this would fail to compile because we
> are binding an rvalue reference to a const object (the array reference is
> decays into a const pointer).
>
> If we used the builtin array type, this would work due to array-to-pointer
> decaying.
>
> Why should the commitee solve this problem?
> *1. One of the long term goals of the committee is trying to make generic
> code easier to write *
> A new defect in the C++ object model was introduced in C++ 17 with
> guaranteed copy elision.
> This problem was not pronounced yet, because it only affects generic code.
> - After concepts becoming wide-spread with the release of C++ 20, generic
> code would be more common.
> - Binding prvalues to universal references would be thought as
> almost-always-a-bug just like "slicing".
> *2. Generic C++ 17 wrapper libraries transitioning to concepts would cause
> a behaviour change in user code.*
> Consider an existing complete wrapper library written in C++ 17.
> It disallows prvalues, as it cannot treat them properly.
> If the library wants to refactor to using concepts instead of ugly
> metaprogramming, then the concept would also have to disallow NonCopyable
> and NonMovable types.
> This would mean the concepts won't be satisfied by these types.
> But if the concept is used elsewhere in a prvalue manner (which is the
> case for the toy example - the matrices use case), then this use case is
> broken, or we fragment our concept hieriechy.
>
> If the concept is a standard library one, or in a different library (which
> will probably become common in a few years from now) then this requires
> forking the concepts library to maintain backward compatibility with this
> use case.
>
> *3. This is a huge broken invariant of the object model*
>
> *4. The proposal aims to add prvalues references, which is one step closer
> to an RAII based object model and guaranteed Ultimate Copy Elision.*
> The unique property of prvalues in the object model is that they are "the
> unique way to refer to their result object in the entire program" (which is
> the core reason for delaying it's creation to when it is actually used).
> This is really a `unique_ptr` to the storage location of the result
> object, where the destructor of the unique_ptr is what starts the lifetime
> of the result object.
> Extending this analogy with further language extensions (`restrict`
> qualifier) would make the language more efficient by default.
>
> *5. This proposal resolves some of the objections to Herb-ceptions
> exception propogation zero-overhead guarantee*
> Throwing an lvalue in the Herb-ception proposal is not guaranteed to be
> zero overhead for the same reason NRVO is not guaranteed by the language in
> any way.
> Throwing a prvalue is OK, and catching by `prvalue` reference does not
> materialize it.
> This is achievable with the correct implementation of `std::error` type
> that treats prvalues differentely then other types.
> In fact, the response to this proposal is
>
> Core proposal - *first class **prvalue references.*
> Strawman syntax (not essential at all, please no bikesheding): `T !&&! t`
> is a prvalue reference of type `T` with alias name `t`.
>
> `t` does not behave like a regular value, as taking it's address is
> forbidden, or assigning it (because it is not a glvalue, but a prvalue) and
> if you want a materialization conversion, you need to case away `pr`-ness
> via `std::move` or a hypothetical `std::materialize`.
>
> Forwarding references would also bind to prvalue references.
>
> Placement new expressions taking `prvalues` reference arguments is the
> only way of the core language to perform a materialization conversion on a
> `prvalue` reference.
>
> `prvalue` references behave like aliases to objects, and they need to be
> the unique owners of the lifetime of their result object (otherwise they
> can't control materialization).
>
> This means that they behave like "`unique_ptr` to the prvalue's lifetime,
> where the destructor materializes them".
>
> To fix the toy example we would need to add
> ```c++
> namespace std
> {
> template<typename T>
> constexpr T && materialize(T !&&! t)
> {
> aligned_storage_t<sizeof(T), alignof(T)> storage;
> T &!!& unique_storage_ref = std::move(new (&storage) T(t));
> return unique_storage_ref; //guaranteed copy/move elision as we return
> a prvalue, and so the above line will be performed on the caller stack frame
> }
> }
>
> template<Addable A>
> struct lazy_addable_wrapper
> {
> //...
> A operator*() !&&! //cv qualifier
> {
> return {*std::materialize(*this)};
> }
> template<MovableOrCopyable B = A>
> A operator*() && //cv qualifier
> {
> //same as before
> }
> template<Copyable B = A>
> A operator*() & = delete;
> ```
>
> Can you spot more new bugs in this code?
>
> Extension Proposal
> Just as move semantics needed to integrate with the language, this new
> type of reference needs a deep integration as well.
> I have an outline, and it also solves the problem of transferring this
> value semantic information, but I won't bother with publishing it in one
> go (again...) as it would cause this to be too large.
> In short, for this `prvalue` reference to integrate well, we need a
> mechanism to transfer the ownership of a `prvalue` just like a return
> statement behaves like a lifetime extension of a prvalue.
>
> In fact, the lifetime extension rules could be reframed in terms of
> prvalue references (and that would convert many dangling reference bugs
> into compile time errors).
>
> This mechanism must be part of the core object model, and it should behave
> similar to the semantics of the `restrict` qualifier in C.
>
> Summary
>
> Handling of prvalues by generic code is a major hole in the object model
> that needs to be fixed.
> The simplest solution is the most direct one - add a reference
> corresponding to this value category.
> This solution is benefitial especially if integrated correctly in the
> language (safer and more efficient language, makes vocabulary types truly
> zero-overhead).
>
>



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