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

[Olaf Meeuwissen]

Hi Povilas, list,

Seeing you assigned the MR referenced below to me, here goes ;-)

# I was going to follow-up anyway but wanted to give other the chance to
# chime in first.

Povilas Kanapickas writes:

> 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.

> As a concrete example of what the proposal would entail I've opened this
> PR which moves genesys backend to C++:
>
> https://gitlab.com/sane-project/backends/merge_requests/61
>
> Total PR excluding generated files and spaces is only ~55 lines: around
> 40 lines of extern "C" definitions in the internal headers and 15 lines
> of changed code in the build system.

I'll follow up with review comments in the MR itself.

> 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.

Think of what your LibreOffice users will say when an uncaught exception
takes their unsaved document(s) with it :-O

# The SANE backend I referenced above tries to give C++ SANE frontends
# a fighting chance by calling `std::terminate()`.

>  - using simple C++ idioms to automatically free memory on function
> exit. This would essentially eliminate 95% of potential sources of
> memory leaks in the backend. I've just fixed one recently, so it's
> indeed an issue.

ACK.  Memory management in C++ is a lot less hassle, compared to C,
assuming you use RAII consistently for any custom types.

Hope this helps,
--
Olaf Meeuwissen, LPIC-2            FSF Associate Member since 2004-01-27
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