[cxx-abi-dev] C++0x POD

[Lawrence Crowl]

On 7/16/09, Mark Mitchell <mark at codesourcery.com> wrote:
> Lawrence Crowl wrote:
> > With all the changes in the standard library, I do not see much
> > value in preserving the ABI of the core language.  Programmers
> > can reasonably expect some change to the ABI in a major release
> > of the standard.
>
> Not when they're not using the new features!
>
> If a user gets a new compiler, compiles their application without
> changes, and can't link with the library they have from a third-party
> provider, I think we've made a mistake.  Of course, I'm fine with the
> idea that using C++0x features causes the ABI to change.

The standard library uses new features of the language heavily.  Most
programs use the standard library, and will therefore use the new
features of the language.

My understanding is that in some cases, compiling existing code with
the new standard library results in calling new functions.  The most
prominent class is the move functions.  Among other things, this change
means that some calls within inline functions in the third-party library
headers will be calling one function in the user's code and another
function inside the library.  That is not binary compatible, independent
of the mangling.  It may work by accident, but I wouldn't guarantee it.

My understanding is that some functions in the C++03 standard library
no longer exist in the C++0x standard library, though the existing call
expressions will continue to work.  Even so, the old third-party library
will not link with the C++0x standard library.  My memory could be wrong
(the library does change frequently), so I will need to verify that
claim.  Anyway, if so, that is not binary compatible.

There are some customers that do not use the standard library, and they
could perhaps avoid an ABI change.  Is protecting those customers worth
the long-term cost of a less efficient ABI?

At the very least, I recommend keeping a list of possibilities and then
if testing shows that the language-and-library combination is not
compatible, going ahead and changing the language ABI.

-- 
Lawrence Crowl