On Wed, Jun 25, 2008 at 4:04 PM, Andrew Haley <[EMAIL PROTECTED]> wrote: > Hans-Peter Nilsson wrote: >>> Date: Tue, 24 Jun 2008 10:36:15 +0100 >>> From: Andrew Haley <[EMAIL PROTECTED]> >> >>> I thought cast-through-pointer-to-union didn't work and was already >>> disallowed; we've been around all this already. >> >> We also bless assignments through unions, and this could be >> argued as assigning through a union, albeit casted. >> >>> This patch of yours >>> already documents uncontroversial behaviour. >> >> That's what I hope, but the existence of that code together in >> an *else* clause of #ifdef YES_ALIAS by a well-known author >> makes it de-facto controversial IMHO. Note also that another >> maintainer thought the code to be valid; see the PR. > > So I see. I'm pretty sure that the compiler's alias analysis won't > think it's valid, but I haven't checked.
Right. It happily "mis-"optimizes it. And on a second thought I agree the construct is invalid. > Do we actually document anywhere that a C++-style type pun along the lines > of > > reinterpret_cast<T&>(x) > > will not work? I'm guessing it probably won't, and that the union trick > is the only thing we do support. This is not a "pun", it only re-interprets the pointer _value_, not what it points to. Richard.
