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Hello Ming,
thank you very much for your quick feedback and I am happy to
hear that the propagation of exceptions works well with my patch!
Regarding your point about exceptions and thr "throw (...)"
specification: You are right in saying that an exception
specification is still an important in programming languages such
as Java. However in Java two kinds of exceptions exist and only
one kind of them (the checked exception) works with exception
specifications:
- Checked exceptions. Checked exceptions are the
kind of exceptions that you are talking about, where the
programmer is forced to somehow handle the exception. Handling
the exception can mean either installing a catch handler or
adding this exception type to the "throws" declaration of your
method. If the exception is not handled, a compile error will be
generated - as you wrote.
- Unchecked exceptions. Unchecked exceptions are
the kind of exceptions that the programmer is not forced to
handle. For this kind of exception, the programmer is not forced
to install a "catch" handler, so no compile error will be
generated if the exception is not handled at all. Such
exceptions also do not appear in the "throws" declaration of the
methods. Unchecked exceptions are all exceptions that derive
from the RuntimeException and Error classes.
For PJSIP a "pj::Error" was mapped to a RuntimeException up to
now (already before my changes), so a "pj::Error" from the C++
PJSIP library raises an unchecked exception. This is true before
and after my changes. This means that - as of now - a Java
programmer is not forced to handle a "pj::Error" from the C++
code. This is because the "pj::Error" will be mapped to an
unchecked exception. The consequence is that there is no compile
error if the user forgets to install a catch handler on the Java
side for errors from PJSIP. The exception for the "pj::Error" also
does not appear in the "throws" declaration of the Java methods
generated by SWIG as only checked exceptions should appear there,
but not unchecked exceptions such as "RuntimeException".
What I want to say in short is that my patch does not change
anything in terms of exception handling on the Java side. The old
behaviour was already to throw an unchecked exception that the
user was not required to catch and the new approach is the same.
And already before my changes we were not using exception
specifications on the Java side as we were using unchecked
exceptions. Or have I missed or overlooked something?
What we might want to consider for the PJSIP Java bindings is to
change the exception handling there to throw checked exceptions if
a "pj::Error" is thrown from the C++ code, in order to force users
to handle a "pj::Error". But this is more or less a separate
change that does not really have anything to do with my changes.
Thanks again for the quick testing and the quick feedback!
Have a nice day and best regards,
Christian
On 16/10/18 06:31, Ming wrote:
Hi Christian,
I did some testing here and the exception does work great during runtime now!
However, in my opinion, the "throw()" specification is still important
for SWIG. While it's true that throw() is deprecated for C++, it
remains a valid (and important) part of other languages such as Java.
And having throw() in the declaration of the function will help the
programmers during the development process.
For example, if you try to call a function which throws an exception
in Java but forget to put try and catch, the compiler will give an
error such as:
sample.java:155: error: unreported exception Exception; must be caught
or declared to be thrown
But it's now gone, so the developer must now carefully check each
function in order to know whether it can throw an exception.
Best regards,
Ming
On Mon, Oct 15, 2018 at 10:18 AM, Ming <ming@xxxxxxxxx> wrote:
Hi Christian,
Thank you for the hard work. Let us review and test this. Given the
scale of the platforms and language binding affected, this may take
some time. We will update you again.
Best regards,
Ming
On Mon, Oct 15, 2018 at 4:19 AM, Christian Hoff <christian_hoff@xxxxxxx> wrote:
Hello all,
during the last weeks, I worked hard on the patch for PJSUA2 and C++ 17
compatibility again. For those who have forgotten: Currently the PJSUA2
headers cannot be compiled with C++ 17 because C++ 17 removes support for
exception specifications. But unfortunately PJSUA2 makes have use of
exception specifications.
The open issue with my last patch was that SWIG no longer generates catch
handlers that catch "pj::Error", as Ming pointed out. I have now updated the
patch so that SWIG also generates the required catch handlers. Doing so has
been a quite difficult, tedious and time-consuming task. But finally I am
done and I am happy to share the resulting patch for inclusion into pjsip.
The updated patch is attached to this mail.
The magic on the SWIG side is done with the "%exception" directive, which
installs a default global exception handler. In my updated patch, I am now
using this "%exception" directive to generate the catch handler for
"pj::Error" (see the new file "pjsip-apps/src/swig/pj_error.i"). The
rationale behind this was that I really wanted to remove all the "throw
(pj::Error)" from the header files, so I did not want to have a "PJ_THROW"
that would expand to nothing during the compile and expand to the old "throw
(...)" when using SWIG. "throw (...)" is now deprecated syntax and we should
not keep it around for the sake of SWIG in my opinion.
Some basic testing was also done:
To verify the C++ 17 compatibility, I compiled my application with the
installed adopted pjsua2 header files.
To verify the generated SWIG language bindings and catch handlers:
For the Python language binding, I verified that the compile works.
Afterwards I installed the generated language binding and ran the test file
"test.py". "test.py" also verifies the exception handling. I also inspected
the generated catch handlers for pj::Error visually.
For the Java and C# bindings, I verified that the compile works and
inspected the generated catch handlers visually. Unfortunately I did not
find a suitable unit test for the exception handling, but I am optimistic
that it works nevertheless. I could not run the generated code for the C#
bindings because I do not have Xamarin installed on my machine. But as I
said, at least I can compile the C++ wrapper code for C#.
I hope that this patch can now be considered for inclusion into PJSIP.
Best regards,
Christian
On 25/09/18 10:26, Max Truxa wrote:
Hello Christian,
how about replacing the throw specifications with a macro that expands
to nothing when compiling as C++ but `throw(Error)` when being parsed
by SWIG?
Something like this:
#ifdef SWIG // defined if SWIG is parsing the file
# define PJ_CAN_THROW throw(Error)
#else
# define PJ_CAN_THROW
#endif
struct Foo
{
void bar() PJ_CAN_THROW;
};
Regards,
- Max
On Tue, Sep 25, 2018 at 8:30 AM Christian Hoff <christian_hoff@xxxxxxx>
wrote:
Hello Ming,
this is also an interesting idea. But will this actually work..? I am
wondering because SWIG only parses the header files of PJSUA2. However
the actual function implementation is in the ".cpp" source files and in
these files you can find the code that throws the exceptions and that
could use "PJ_THROW()". But to my knowledge SWIG does not parse the
source files and would thus not know which functions may call
"PJ_THROW". Or am I wrong about this?
Best regards,
Christian
On 25/09/18 03:08, Ming wrote:
Hi Christian,
We haven't found a good way either. Our idea is to put PJ_THROW()
macro to replace throw(), which can somehow be used by SWIG to
generate the exception specs.
Regards,
Ming
On Tue, Sep 25, 2018 at 4:21 AM Christian Hoff <christian_hoff@xxxxxxx>
wrote:
Hello Ming and all,
I have now done some research into SWIG and exceptions. After reading
the SWIG documentation, I found out that SWIG indeed parses the C++
exception specification to figure out for what types of exceptions
"catch(..)" should be generated (see
http://www.swig.org/Doc3.0/SWIGDocumentation.html#SWIGPlus_exception_specifications
). Exception types that are specified in the exception specification are
automatically propagated to the higher language (Java, C# etc.). Now
that we need to remove the exception specification, we lose that
capability. Thanks to Ming for the hint. However there is the
possibility to adapt the SWIG interface file
("pjsip-apps/src/swig/pjsua2.i") and to add explicit "%catches"
statements for each method that describe which types of exceptions the
method could possibility throw. So I guess I need to rework my patch for
this. I just hope that I will find some way to avoid having to annotate
every method with "%catches".... I'll see. Or if someone knows how to do
it, then I would be very grateful for a reply.
Best regards,
Christian
On 24/09/18 12:43, Christian Hoff wrote:
Hello Ming,
thanks for your reply! I will have a look into the SWIG topic this
evening and will let you know my findings.
Best regards,
Christian
On 24/09/18 09:03, Ming wrote:
Hi Christian,
We apologize for the delay in response. We're currently reviewing it
and have also managed to do some testing. While for C/C++, this seems
to be okay, we're still considering how to best handle this for our
Android and Java (via SWIG binding), which may still need the "throw"
specifications in order for the developers to know that the functions
may throw some exceptions.
If you have any feedback regarding this, please let us know.
Best regards,
Ming
On Mon, Sep 24, 2018 at 2:32 PM Christian Hoff
<christian_hoff@xxxxxxx> wrote:
Hello all,
some feedback for this patch is still appreciated. In general I am
getting somewhat frustrated about the collaboration with people who
want
to contribute to the project. Already in the past other people have
posted patches here to this list, but did not get any response - not
even a response what needs to be changed for the patch to be
accepted. I
think this is not a good mode of collaboration. Everyone who posts a
patch here to this mailing list has done this with the intention to
help
the project PJSIP to move along (and has spent much time developing the
patch), so you should at least return the favour and review the patch.
This particular issue that I am trying to address with my patch (that
the PJSUA2 headers don't compile against C++17) is an issue that you
will eventually need to fix anyhow, so why not fix it right now? As of
now, modern versions of GCC use C++ 14 by default, it is only a matter
of time until C++ 17 will become the default and people will start
complaining that there are compile errors from the PJSUA2 headers.
Besides this, my own app uses PJSUA2 with C++ 17 features, so I would
have to revert back to C++14 if this patch is not accepted upstream.
This would be really unnecessary. So I would really appreciate a patch
review.
Best regards,
Christian
On 18/09/18 20:57, Christian Hoff wrote:
Hello all,
Hello Ming & Riza,
do you have any feedback regarding this patch for C++ 17 support? I am
also willing to rework it if required, but some feedback would be
nice.
Thanks in advance and kind regards,
Christian
On 12/09/18 18:23, Christian Hoff wrote:
Hello all,
currently it is not possible to compile a pjsua2 application with
C++17. The reason is that C++17 removes support for dynamic exception
specifications (that is "throws ..." declarations at the end of a
function). Unfortunately PJSUA2 makes heavy use of exception
specifications and most PJSUA2 functions are declared as "throw
(Error)" - meaning that they can throw an exception of type "Error"
that the programmer is supposed to catch.
The consequence is that a lot of compile errors can be observed when
compiling a pjsua2 application with C++17. The compile errors look as
follows:
/usr/local/include/pjsua2/json.hpp:80:33: error: ISO C++17 does not
allow dynamic exception specifications
virtual string saveString() throw(Error);
^~~~~
The compile of the same application worked fine when using the C++14
standard, but it got broken when I switched from C++14 to the C++17
standard.
See
https://www.bfilipek.com/2017/05/cpp17-details-fixes-deprecation.html#removing-deprecated-exception-specifications-from-c17
and
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0003r0.html
for more background about the removal of exception specifications.
To make the application compile again, I had to remove "throw(Error)"
in the whole PJSUA2 source code and include files. This seems to be
the only viable and good fix for this issue. I wrote my own "sed"
script to do that and made some cosmetic corrections afterwards. Then
I generated the attached patch file. Could you please apply this
patch to the code base or give me feedback what I still need to
improve? I verified that pjsip compiles fine with the patch applied.
Thank you very much!
Best regards,
Christian
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