On Wed, 30 Oct 2019, Jonathan Wakely wrote:
We have two options:
1) Make the base class the same as it was in C++98, and override it
for other modes. The program above would always print 24. This makes
the layout always consistent with the GCC 4.6 layout, for all modes,
but incompatible with the default -std=gnu++14 mode since GCC 6 (and
incompatible with code explicitly using C++11 or C++14 in GCC 4.7 and
above).
Or:
2) Always use the new base class, and override it for C++98. The
program above would always print 16. This makes layout for C++98
compatible with the current -std=gnu++14 default, but no longer
compatible with C++98 code from previous releases.
3) Don't use any base class, write the 5 typedefs manually.
N.B. the decision should not be "choose the option that makes
sizeof(J) smaller", because I can easily create an example where C++98
prints 16 and other modes print 24, so we're not trying to optimize
for size. What matters is compatibility, not the absolute value.
3 is never bigger ;-)