[PATCH] "Fancy" ARBs

[hfinkel at anl.gov]

In http://reviews.llvm.org/D9714#171426, @hfinkel wrote:

> In http://reviews.llvm.org/D9714#171408, @rsmith wrote:
>
> > First off: I'm not happy about having this extension in upstream clang until we have a strong indication that this is the direction that will end up standardized. For now, I'd recommend maintaining this as a clang fork on github or similar.
>
>
> Will do.
>
> > With that said, I'm going to review this as if for upstream clang.
>
>
> Thanks! (The point of doing this is to get feedback on implementation-related issues)
>
> > I don't like that you create two variables here. We try to maintain as much source fidelity as we can, and I think we can do better here -- how about instead introducing a new form of expression that represents "stack-allocate a certain amount of memory" (with a subexpression for the initialization, if you allow these variables to have an initializer), much like MaterializeTemporaryExpr does for a SD_Automatic temporary, but parameterized by an expression specifying the array size? Then create just a single expression of the std::arb type, initialized by that expression.
>
> > 
>
> > You should also introduce a Type subclass to represent type sugar for the ARB type, so that we can model that int[n] desugars to std::arb<int> but should be pretty-printed as an array type.
>
>
> That makes sense (and this is very similar to how we currently handle std::initializer_list).
>
> Any opinion on:
>
> 1. Should we bother with the access-control-overriding init type? We could just make the constructor public but make it UB if the user calls it (it would be implementation-specific anyhow).
> 2. Should I make std::arb manage the lifetime of the objects directly? If it just takes a special allocation expression maybe that's more natural? I'd like to not force extra work for POD types (but I imagine I could use some enable_if/is_pod logic to leave PODs uninitialized).


Or to put it another way, should the aforementioned new "stack-allocate a certain amount of memory" expression also be responsible for calling the constructors of non-POD array elements and register calling their destructors as cleanup, or should that logic be embedded in std::arb?

> 

> 

> 2. The automatically-included header (or similar) with a simpler class, or just requiring the header and making the class more fully-featured?

> 

>   Thanks again!



http://reviews.llvm.org/D9714

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