Support compilation with C++11 flags

[PeterBowman]

All Travis CI cron jobs pull YARP's devel branch (currently at v2.3.71 (unstable)), which causes them to fail:

https://travis-ci.org/asrob-uc3m/robotDevastation/builds/291473501

Note that future YARP releases (2.3.72 onwards) will need C++11 for using YARP, not just compiling it:

https://github.com/robotology/yarp/blob/31a9e88/doc/release/v2_3_72.md#important-changes

[PeterBowman]

From cmake/YarpSystemCheck.cmake at current YARP devel (2.3.71, ref):

set(CMAKE_CXX_EXTENSIONS OFF) set(CMAKE_CXX_STANDARD 11) set(CMAKE_CXX_STANDARD_REQUIRED ON)

On my PC (Ubuntu 16.04), this adds a -std=gnu++11 to client code. On Travis (Ubuntu 14.04), such option is not recognized (ref) and compilation fails.

https://cmake.org/cmake/help/v3.1/prop_tgt/CXX_STANDARD.html

[PeterBowman]

On Travis (Ubuntu 14.04), such option is not recognized (ref) and compilation fails.

Not anymore! Trusty images on Travis now use gcc 4.8.4 (ref). In fact, I already pointed this out on https://github.com/roboticslab-uc3m/gait/issues/6#issuecomment-292308644, which means that this comment was clearly outdated.

Closing, a recent CMake code review (#105) fixed this.

[PeterBowman]

Note to self, this is currently set at libYARP_OS/CMakeLists.txt (devel branch, ref):

if(CMAKE_VERSION VERSION_LESS 3.1)
  if(DEFINED CXX11_FLAGS)
    target_compile_options(YARP_OS PUBLIC ${CXX11_FLAGS})
  endif()
else()
  target_compile_features(YARP_OS PUBLIC cxx_nullptr
                                         cxx_override
                                         cxx_rvalue_references)
endif()

https://cmake.org/cmake/help/v3.1/command/target_compile_features.html

[PeterBowman]

Another note to expand on what's happening under the hood. This is an excerpt from YARPTargets.cmake when configured on a system with CMake 3.1+ (supports target_compile_features):

# Create imported target YARP::YARP_OS
add_library(YARP::YARP_OS SHARED IMPORTED)

set_target_properties(YARP::YARP_OS PROPERTIES
  INTERFACE_COMPILE_FEATURES "cxx_nullptr;cxx_override;cxx_rvalue_references"
  INTERFACE_INCLUDE_DIRECTORIES "/home/bartek/git/yarp/src/libYARP_OS/include"
  INTERFACE_LINK_LIBRARIES "YARP::YARP_conf"
)

When built with CMake <3.1, it's smart enough to define INTERFACE_COMPILE_OPTIONS instead of INTERFACE_COMPILE_FEATURES, setting (depends on the compiler, I guess) -std=c++11. In fact, I did see a std=gnu++11 option propagated down to the RD targets.

See also: https://cmake.org/cmake/help/v3.1/manual/cmake-compile-features.7.html.