[cfe-dev] question about OpaqueValueExpr (was: injecting a member function)

[John McCall]

On Feb 22, 2011, at 1:37 AM, Eric Niebler wrote:
> On 2/19/2011 1:18 AM, John McCall wrote:
>> On Feb 18, 2011, at 1:49 AM, Eric Niebler wrote:
>>> On 2/18/2011 3:41 PM, John McCall wrote:
>>>> let tmp = obj in
>>>> tmp.setProp(tmp.getProp() + 1)
>>> 
>>> In practical terms, that means that, when evaluating tmp, evaluate obj
>>> (once), and thereafter all other times tmp is evaluated, just use the
>>> result already computed. And this Just Works wrt lvalues and rvalues.
>> 
>> Right.  When creating the OVE, you give it a type, value kind, and
>> value object kind matching the expression it gets bound to.
> 
> Here's a question. What if I create a OpaqueValueExpr, and I want it to
> be an lvalue (so I can mutate it in place), but the expression it gets
> bound to is actually an rvalue? Will that work? Or can I cast an rvalue
> to an lvalue somehow and bind the OVE to that?

OVEs need to match their underlying values perfectly, including in value kind.

> The reason I ask is because I want this:
> 
>  a.b += 42
> 
> to be rewritten as:
> 
>  let tmp = a
>  in tmp.set(tmp.get() += 42)
> 
> tmp.get() will (probably) be an rvalue, but for feature parity with MS,
> I need to use += instead of the (obvious and simpler) +.

Does this really need to be "tmp.set(tmp.get() += 42)"?  I would expect it
to just be "tmp.get() += 42" if tmp.get() happens to yield an lvalue.

Regardless, you need to be testing for this and building different ASTs
around it rather than trying to force invalid ASTs.

John.