Fails to compile with GCC 6.x - make destructors noexcept

[breznak]

Testing the latest & greatest gcc compiler, I run into some issues.

• about the use of NTA_CHECK(..) << "msg" macro:

[ 40%] Building CXX object src/CMakeFiles/nupic_core_solo.dir/nupic/engine/Region.cpp.o
/home/mmm/devel/HTM/nupic.core/src/nupic/engine/Output.cpp: In destructor ‘nupic::Output::~Output()’:
/home/mmm/devel/HTM/nupic.core/src/nupic/engine/Output.cpp:51:36: error: throw will always call terminate() [-Werror=terminate]
   NTA_CHECK(links_.size() == 0) << "Internal error in region deletion";
                                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/mmm/devel/HTM/nupic.core/src/nupic/engine/Output.cpp:51:36: note: in C++11 destructors default to noexcept

Looks like we'll have to rewrite these macros

[vitaly-krugl]

I think this makes sense. We generally don't want the possibility of an exception within the scope of a destructor. Good read on the subject: http://bin-login.name/ftp/pub/docs/programming_languages/cpp/cffective_cpp/MEC/MI11_FR.HTM

[breznak]

So, how do we do it? Compile with Wno-error=terminate? Or better yet something like #pragma push to silence it only for the NTA_CHECK/ASSERT macros?

[vitaly-krugl]

We should have a common approach for all C++ destructors. Effective C++ talks about two problems concerning exceptions in destructors:

1. terminate in case exception from a destructor flows to another destructor in higher scope
2. incomplete destruction/release of resources in case of exception in destructor

I think that we should either avoid using anything in a destructor that could raise an exception or wrap code in a simple try with an empty-bodied catch as recommended in http://bin-login.name/ftp/pub/docs/programming_languages/cpp/cffective_cpp/MEC/MI11_FR.HTM.

Calling NTA_CHECK from a destructor would be an oxymoron. Perhaps something like NTA_WARN_NO_EXC (doesn't exist yet) would be a more appropriate alternative for this specific scenario.