[cxx-abi-dev] Key functions and templates

[Zbigniew Sarbinowski]

Lawrence Crowl <crowl at google.com> wrote on 02/22/2010 04:50:46 PM:
> On 2/22/10, Mark Mitchell <mark at codesourcery.com> wrote:
> > Alasdair Grant wrote:
> > > Implementations (at least of the ARM variant of the ABI) seem
> > > to differ about whether the code above should define the vtable
> > > for G_key_nodef<int>.  GCC 4.4 for ARM does define the vtable -
> > > which suggests it is not treating G_key_nodef<int>::f() as a
> > > key function (if it did, it could rely on some other unit to
> > > be defining f(), and hence the vtable).
> >
> > Given that there is vague linkage involved, how does this matter?
> > It sounds to me like GCC is being wasteful, in that it's defining
> > a vtable not needed in this object file, and guaranteed to be
> > present anyhow, but as vague linkage will collapse the definitions,
> > it sounds like this is "just" a waste of space in object files,
> > not in final executables.
> >
> > Even if -- as on Symbian, where I suspect this question arises :-)
> > -- the waste makes it into a DSO, how is that an ABI conformance
> > problem?
>
> How about the following scenario?
>
>   A translation unit declares an "extern template" specialization
>   of G_key_defined<int>::f().  The compiler infers that the virtual
>   table need not be generated even though the translation unit
>   otherwise instantiates G_key_defined<int>.  The specialization of
>   f() is not subsequently used, and so the vtable will fail to exist.

I have a related issue for which I would like to get clarification.
There seems to be a contradiction between ABI and C++ Standard.
Given the following.

/* t1.h */
class A
{
 virtual int foo() = 0;
 };

 struct B : public A
 {
 int foo();
 };

/* t1.cpp */
#include "t1.h"
inline int B::foo() { return 55; }

/* t2.cpp */::
#include "t1.h"
int main()
{
 B b; // this line forces to generate vft
  return b.foo();
}

The g++ compiler will compile and link fine since B::foo() will be
generated as part of t1.o.
On the other hand, XL (IBM) compiler comes back with "Undefined
symbol: .B::foo()".  This is because
t1.o does not have its definition.  It was optimized out since XL didn't
find any reference to it in t1.cpp.

According to C++ Standard the above TC is invalid because it violates the
last sentence from 3.2.3:
"An inline function shall be defined in every translation unit in which it
is used."

However C++ABI by last sentence in sec. 5.2.3 (Virtual Tables) makes this
TC valid.
"Note that if the key function is not declared inline in the class
definition, but its definition later is always declared inline, it will be
emitted in every object containing the definition."
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