Re: flyweights, nested flyweights and shared libraries

From: Andreas Buykx (A.Buykx_at_[hidden])
Date: 2021-02-23 09:59:50

Hello Joaquín,

Thanks for your help. This morning it dawned on me what you meant with the remark about the destruction: I implemented all constructors as explicit default in the cpp-file but I had forgotten to do that for the destructor. Now everything runs like a breeze, except for one situation where I get an assertion:

boost::flyweights::detail::recursive_lightweight_mutex::scoped_lock::scoped_lock(boost::flyweights::detail::recursive_lightweight_mutex &): Assertion `pthread_mutex_lock(&m_)==0' failed.

This exception is raised while destroying the nested flyweight wrapper from the nesting flyweight wrapper, but only in one of my many test cases where these flyweight wrappers are used. Do you have any suggestions on why this might happen?

Regarding the use of intermodule_holder I mistook a link error for a compile error:

/opt/rh/devtoolset-6/root/usr/libexec/gcc/x86_64-redhat-linux/6.3.1/ld: Test/appz/ppf/unittests/model/results/CMakeFiles/unittest_PpfModelResultUnitTest.dir/PpfResultCaseUnitTest.cpp.o: undefined reference to symbol 'shm_unlink@@GLIBC_2.2.5'
//lib64/librt.so.1: error adding symbols: DSO missing from command line

All that aside: you mention that getting intermodule_holder is the preferred option, but given that it's "... intermodule_holder is considerably more onerous than static_holder in terms of compilation times and introduces a non-negligible overhead at program start-up ...", isn't wrapping the flyweight in an accessor class (especially your templated construct) at least as good an option that maybe deserves a mention in the documentation?

Thanks again for your help,
Andreas

From: Boost-users <boost-users-bounces_at_[hidden]> On Behalf Of Joaquin M López Muñoz via Boost-users
Sent: zaterdag 20 februari 2021 16:22
To: boost-users_at_[hidden]
Cc: Joaquin M López Muñoz <joaquinlopezmunoz_at_[hidden]>
Subject: Re: [Boost-users] flyweights, nested flyweights and shared libraries

El 19/02/2021 a las 16:54, Andreas Buykx via Boost-users escribió:
I want to use boost::flyweight to capture some structures containing const data.

The documentation states that if flyweights are exported across shared library boundaries
that the intermodule holder should be used.
Since my flyweights are all created in the same shared library I would prefer to use the
static holder but I keep getting crashes.

It is not only necessary that all flyweights are created in the same DLL: they must also
be destroyed within that DLL. I guess that's the problem you're experiencing.
I tried the intermodule holder but I'm having trouble compiling my sources with that.
Can you provide more info on the kind of problems you're running into?
intermodule_holder tests successfully in Linux, Mac and Windows platforms.
I presume it is Windows you use.

 So my thinking was to wrap all flyweight types in accessor classes which are exposed instead, and the
accessor classes create the flyweights within the same shared library to store the data.
Would that be a valid approach?

I think your first option should be to make intermodule_holder work. That said, the
approach you outline below in your mail looks OK, as the lifetime of flyweight objects
is handled entirely within DLL-specific exported functions. A simpler approach would be
to define a wrapper around boost::flyweight:

// template class to be instantiated *only*
// within your exporting DLL
template<typename T>
struct DLLEXPORT dll_flyweight:boost::flyweight<T>
{
  using boost::flyweight<T>::flyweight;
};

Also, is it possible to have nested flyweights (when they are wrapped by accessor classes)?
And would that cause any additional problems in the above scenario?

I don't see any problem with that.

Best,

Joaquín M López Muñoz