[cfe-dev] Query regarding global value initialization standard

[Eli Friedman]

On Wed, Sep 25, 2013 at 3:02 PM, Karen Shaeffer <shaeffer at neuralscape.com>wrote:

> On Wed, Sep 25, 2013 at 02:40:37PM -0700, Eli Friedman wrote:
> > On Wed, Sep 25, 2013 at 4:49 AM, Karthik Bhat <
> blitz.opensource at gmail.com>wrote:
> >
> > > Hi All,
> > > I was going through a gcc TC for C++11. The test case is as follows -
> > >
> > > // { dg-options -std=c++0x }
> > > // { dg-do run }
> > >
> > > extern "C" void abort ();
> > > extern int ar[2];
> > >
> > > int f()
> > > {
> > >   int k = 0;
> > >   if (ar[0] != 42 || ar[1] != 0)
> > >     abort();
> > >   return 1;
> > > }
> > >
> > > int i = f();
> > >
> > > int ar[2] = {42,i};
> > >
> > > int main()
> > > {
> > >   return 0;
> > > }
> > >
> > > During dynamic initialization of i in function f() the value of ar[0]
> is 0
> > > in case of clang were as in case of gcc it is 42.
> > >
> > > As per standard(section 3.6.2) all global values should initially be
> zero
> > > initialized followed by const initialized if possible before dynamic
> > > initialization takes place.
> > > Hence as per standard the const initialization of int ar[2] = {42,i};
> > > should fail as i is not a const here( which seems to be happening in
> > > clang). Hence ar[0],ar[1] is still zero initialized because of which
> during
> > > dynamic initialization in f() ar[0]  is 0 which seems to be the correct
> > > behavior.
> > >
> > > Can i conclude here that clang is behaving correctly and the tc is
> wrong?
> > > or am i missing something which this gcc tc wanted to capture?
> > >
> >
> > As far as I can tell, your analysis is correct.
> >
> > -Eli
>
> Hi,
> But isn't function f a constexpr? It doesn't modify anything and always
> returns 1.
>
>
It doesn't matter what the implementation of "f" is; "f()" is still not a
constant expression.  Please read [basic.start.init] and [expr.const] in
the C++ standard.

-Eli
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