[sane-devel] Using C++ in sane backends

[Povilas Kanapickas]

On 2019-05-12 07:32, Olaf Meeuwissen wrote>> On 2019-05-09 11:56,
Povilas Kanapickas wrote:
>>> I would propose we allow C++, but that its use must not affect API or
>>> ABI in any way. There should be a hard requirement that there must be no
>>> visible changes outside a specific backend, except the need to link the
>>> backend with libstdc++.
> 
> Technologically, there is no reason why one cannot use another language
> than C.  All the SANE API specification requires is that SANE backends
> provide a certain C API.  What a backend does "under the covers" is that
> backends business.
> 
> I have implemented a third party SANE backend in C++ myself.  See
> 
>   https://gitlab.com/utsushi/utsushi/blob/master/sane/backend.cpp
> 
> Whether we want to allow C++ in the SANE Project's sane-backends (and to
> what extent) is an issue that we need some consensus on, I think.

Agreed. The reason why I thought that it would be possible to achieve
some consensus at all was that there's no consistent design of the
backends included within the sane-backends repository so they could be
considered separate projects within a monorepo. In that case using C++
in one backend is not a problem as most SANE developers will not need to
interact with that code in any way.

>> Two benefits that would yield almost immediate improvements are the
>> following:
>>
>>  - ability to use exceptions instead of manual error handling which
>> currently often explodes the amount of code by 4 times.
> 
> I was "afraid" of this ;-)
> 
> Please be aware that your backend will have to catch *all* exceptions,
> even the ones you don't throw yourself, e.g. an `std::bad_alloc`.  Any
> uncaught exception will crash SANE frontends right then and there with
> no chance of saving state.

Agreed. My plan was to catch all exceptions, print something to logs and
then return some error status code so that everything works just like if
the C implementation got an error it couldn't recover from. The caller
code should be completely isolated from what happens within the backend.

Regards,
Povilas