[cfe-commits] [patch] Bug 5077

[Dimitri Tcaciuc]

Douglas,

Thanks for replying!

On Mon, Oct 12, 2009 at 9:24 AM, Douglas Gregor <dgregor at apple.com> wrote:

>> * (?) There should be no meaningful temporaries left after member
>> initializer completes.
>
> Temporaries can live after the member initializer completes if they were
> bound to a reference member (12.2p5). In this case, the temporaries are
> destroyed at the end of the constructor.

Ah, ok. Although that doesn't stop the reference from being invalid
after constructor completes, does it? Are we throwing a warning about
it somewhere?

Hmm, so then we need to add a clean-up bit for those somewhere inside
CodeGenFunction::GenerateCode after EmitStmt(S); and before
FinishFunction(). I'm trying to think if we can safely assume that
whatever temporaries are still left over at the end of constructor
body before FinishFunction() are safe to discard. I remember that was
the first incorrect fix for this bug that I tried.

There's also the question about how to separate the reference bound
temporaries from the rest. I'm looking at
CodeGenFunction::EmitReferenceBindingToExpr, so I guess I can try

  CXXBaseOrMemberInitializer::const_arg_iterator arg =
Member->const_arg_begin();
  if (const CXXBindTemporaryExpr* TE = dyn_cast< CXXBindTemporaryExpr >(*E)) {
    ...
  }

However, if I make the reference member const and initialize it with a
non const, then the CXXBindTemporaryExpr is wrapped with  an
ImplicitCastExpr and the above plan is foiled..

Also, all live temporaries are on one stack... perhaps take the
reference bound ones off the top, keep in a separate vector, then push
back onto LiveTemporaries at the end of EmitCtorPrologue? Kind of
hacky...

A side question, looking at "This is really ugly" comment in
EmitCtorPrologue, it seems that the special case for references differ
from regular EmitLValueForField only by the absence of CreateLoad and
qualifier assignment. What gives?


Dimitri.