C++ 'extern inline' magic possible?

[Jonathan Wakely]

On 1 March 2011 03:53, Ian Lance Taylor wrote:
> Jonathan Wakely <jwakely.gcc@gmail.com> writes:
>
>> On 1 March 2011 00:25, Ian Lance Taylor  wrote:
>>> "Kevin P. Fleming" <kpfleming@digium.com> writes:
>>>
>>>> I would like to come up with some construction like the 'extern
>>>> inline' that GCC supports for C mode, so that a.h could contain the
>>>> declaration *and* definition of 'bar', allowing code that includes a.h
>>>> to have 'bar' be inlined if the compiler chooses to do so (and leave
>>>> an external reference to 'bar' if necessary so that the version built
>>>> from a.cpp will be used). So far my attempts have only resulted in
>>>> various re-definition or re-declaration errors.
>>>
>>> There is no equivalent to GNU C's "extern inline" in C++.  By the way,
>>> "extern inline" is now actually known as __attribute__ ((gnu_inline)),
>>> as C99 defines "extern inline" to mean something different.
>>>
>>> In C++ you can simply define the function inline in a.h, and not define
>>> it at all in a.cpp.  The right thing will happen.
>>
>> That will work in practice, but it's technically an ODR violation.
>
> No, it's not (there may be a misunderstanding somewhere).  I am
> suggesting that there should be only one definition: the one in a.h.
> That is OK if the definition has inline linkage.  It is certainly not an
> ODR violation, as there is only one definition.

Yes sorry, I did misunderstand.  But in that case a.h must be included
by all callers of the function.  I thought the point was some callers
don't want to include the definition of the class and the function
that uses it.