GCC 6.1 compilation error

[taranco]

When attempting to compile nupic.core with gcc 6.1 the following error occurs:

/home/rts/src/nupic.core/src/nupic/engine/Output.cpp: In destructor 'nupic::Output::~Output()':
/home/rts/src/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/rts/src/nupic.core/src/nupic/engine/Output.cpp:51:36: note: in C++11 destructors default to noexcept
cc1plus: all warnings being treated as errors
make[2]: *** [src/CMakeFiles/nupic_core_solo.dir/build.make:1288: src/CMakeFiles/nupic_core_solo.dir/nupic/engine/Output.cpp.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [CMakeFiles/Makefile2:765: src/CMakeFiles/nupic_core_solo.dir/all] Error 2
make: *** [Makefile:150: all] Error 2

Using the flag -Wno-error=terminate seems to solve the problem. Nupic.core compiles an pass the tests. I suggest take this into account (CommonCompilerConfig.cmake) when this version of gcc is detected.

[vitaly-krugl]

@taranco, could you elaborate on the build environment, including version of nupic.core, the operating system and its version? Thank you.

[vitaly-krugl]

I see, it's a feature of GCC6. I came across this description:

-Wterminate (enabled by default) warns when a throw will immediate result in a call to terminate like in an noexcept function. In particular it will warn when something is thrown from a C++11 destructor since they default to noexcept, unlike in C++98 (GCC6 defaults to -std=gnu++14)."

in https://gnu.wildebeest.org/blog/mjw/2016/02/15/looking-forward-to-gcc6-many-new-warnings/.

See also http://en.cppreference.com/w/cpp/language/noexcept_spec

[vitaly-krugl]

The culprit function is the destructor:

Output::~Output()
{
  // If we have any outgoing links, then there has been an 
  // error in the shutdown process. Not good to thow an exception
  // from a destructor, but we need to catch this error, and it 
  // should never occur if nupic internal logic is correct. 
  NTA_CHECK(links_.size() == 0) << "Internal error in region deletion";
  delete data_;
}

C++ has no mechanism for passing an exception out of a destructor (the GCC6 runtime calls terminate() instead), thus exceptions raised inside a destructor look questionable to the compiler.

[taranco]

It's Arch Linux (GNU/Linux) @vitaly-krugl:

Linux   4.6.3
GCC     6.1.1 20160602
Cmake   3.5.2
Make    4.2.1

Nupic.core     0.4.5.dev0 (Last from git)