Re: [PATCH v2] libstdc++: Add hexfloat/defaultfloat io manipulators.

[Luke Allardyce]

 Thanks, I was wrong about that.

 Then I think we should just bite the bullet and provide the new
 behaviour. If we do have an abi_tag on those types in the next release
 then we can preserve the old behaviour in the old ABI and use the
 C++11 semantics for the abi_tagged type, which will be used for both
 C++03 and C++11 code. I am not too concerned that people who use a
 meaningless modifier in C++03 code get the C++11 behaviour. If they
 really want %g or %G then they shouldn't use fixed|scientific.

Does that mean abi_tag will be enabled with separate compiler flag /
define rather than checking against the __cplusplus value?

Re: [PATCH v2] libstdc++: Add hexfloat/defaultfloat io manipulators.

[Luke Allardyce]

 Also the old standard seems to require that ios_base::fixed |
 ios_base::scientific (or any other combination) falls through to the
 uppercase test; I was trying to use abi_tag for a solution as not only
 would two versions of _S_format_float be necessary, but also num_get
 due to the pre-instantiated templates for char and wchar, which
 led me to http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60642. It might
 just be more trouble than it's worth.

 I don't think we need to worry about that, if I understand correctly
 the combination of fixed|scientific has unspecified behaviour in
 C++03, so we can make our implementation do exactly what it does in
 C++11.

It seems to me that it is well defined, going from  [lib.facet.num.put.virtuals]

 6 All tables used in describing stage 1 are ordered. That is, the first line 
 whose condition is true applies.
 A line without a condition is the default behavior when none of the earlier 
 lines apply.

So fixed|scientific would be equivalent to specifying neither
according to table 58, and the resulting specifier would be %g or %G
depending on whether uppercase is set or not.