[cfe-commits] r148348 - in /cfe/trunk: include/clang/Sema/Sema.h lib/Sema/Sema.cpp lib/Sema/SemaDeclCXX.cpp

[Eli Friedman]

On Tue, Jan 17, 2012 at 4:31 PM, Sebastian Redl
<sebastian.redl at getdesigned.at> wrote:
> Also, I believe the specification of the lifetime of the backing array to be highly defective. [dcl.init.list] says:
> "Otherwise, if T is a specialization of std::initializer_list<E>, an initializer_list object is constructed as described below and used to initialize the object according to the rules for initialization of an object from a class of the same type."
> Note that it says that an object is constructed and then basically copied to the actual object. Below, it says:
> "The lifetime of the array is the same as that of the initializer_list object."
> So if a temporary is constructed, wouldn't the lifetime of the array be that of the temporary?
> The example contradicts this, of course, since that would be stupid, but if the lifetime of the array is the same as that of the actual initializer_list object, then how do you implement this?

I agree; I don't see how the example can possibly work without either
some std::initializer_list-specific magic in the compiler or a library
implementation of std::initializer_list that looks nothing like the
one currently in libc++.  (I think the compiler could actually follow
the standard if std::initializer_list had an appropriate move
constructor and destructor which destroyed the elements, and the
associated memory somehow had block-scope lifetime.)

> auto listptr = new std::initializer_list<int>{1, 2, 3, 4, 5};
>
> Or for that matter, since it's equivalent:
> auto listptr = new auto{1, 2, 3, 4, 5};

"warning: allocating an std::initializer_list with new likely leads to
undefined behavior.  The simplest way to get the behavior you clearly
want with the C++ standard library is 'auto listptr = new
std::array<int,5>{{1,2,3,4,5}}'."

Or we could special-case this somehow to actually request the right
amount of memory from operator new, but that seems to be way outside
anything the standard suggests we can or should do.

-Eli