[cfe-dev] Following up on C++11 final and undefined-reference linker errors

[John McCall]

On Nov 29, 2011, at 7:35 PM, Eli Friedman wrote:
> On Tue, Nov 29, 2011 at 7:20 PM, John McCall <rjmccall at apple.com> wrote:
>> On Nov 29, 2011, at 5:34 PM, Jeff Walden wrote:
>>> http://lists.cs.uiuc.edu/pipermail/cfe-dev/2011-November/018779.html
>>> 
>>> A week ago I asked if adding |final| to a class causing linker errors could possibly not be a compiler bug; replies suggested it likely would be one.   Having reduced it to the code at the end of this mail, I'm pretty sure it's not one.  [expr.reinterpret.cast]p7 says reinterpret_cast<> on a class with virtual functions, or to a class which is not an actual type of the instance being cast, has undefined behavior.  If I read this right, failure to link is permissible behavior.  (GCC 4.7 also produces a linking error.)
>> 
>> Only if the undefined behavior is provably reached.  Undefined behavior only taints execution paths which reach the undefined behavior.
>> 
>> Your program, however, is outright ill-formed, with no diagnostic required, because it contains the declaration of a virtual function (Wrapper::hidden()), but nowhere in the program is there a definition for it.  All virtual functions are considered "used" and therefore must be defined, even if their classes are never even mentioned elsewhere in the program.
> 
> hidden() isn't really relevant here: we end up with the same behavior
> even if you completely remove it from the testcase.

You're right;  I've misread the testcase.

I am still totally comfortable with unconditionally rejecting the definition of a final abstract class, however.  Similarly, we should reject final pure virtual functions, although the error there is much more obvious and thus less likely to occur in practice.

John.