Enable c++11 support through CMake functions.

[hgaiser]

This PR enables C++11 support using CMake variables (CMAKE_CXX_STANDARD for choosing the standard and CMAKE_CXX_STANDARD_REQUIRED for disabling CMake's default functionality of "decaying" to c++98 if c++11 is not available).

These variables are added since CMake 3.1.13 so a bump of the minimum required cmake version was necessary.

[Yangqing]

Thanks so much for this!

A quick check: http://packages.ubuntu.com/trusty/cmake cmake on 14.04 is actually still 2.8, so do you think it is possible for us to provide a backward compatible path for folks who have an older cmake?

[facebook-github-bot]

@hgaiser updated the pull request - view changes

[hgaiser]

Thanks so much for this!

A quick check: http://packages.ubuntu.com/trusty/cmake cmake on 14.04 is actually still 2.8, so do you think it is possible for us to provide a backward compatible path for folks who have an older cmake?

Ah .. 14.04 is a target platform for caffe2? I suppose it makes sense since it's a LTS, but it can be annoying in cases such as this. I suppose there are two options then, merge this PR and provide people a guide to upgrade cmake in Ubuntu 14.04 or don't merge this PR and keep it the way it was.

ps. I just updated this PR to also set the CMAKE_C_STANDARD variable.

[Yangqing]

@hgaiser I found that one can potentially use this to do cross version support. Would you like to give that a shot?

if(CMAKE_VERSION VERSION_LESS "3.1")
include(CheckCXXCompilerFlag)
CHECK_CXX_COMPILER_FLAG("-std=c++11" COMPILER_SUPPORTS_CXX11)
if(NOT COMPILER_SUPPORTS_CXX11)
MESSAGE(FATAL_ERROR "Your compiler does not support c++11")
endif()
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
else()
set(CMAKE_CXX_STANDARD 11)
endif()

[hgaiser]

@hgaiser I found that one can potentially use this to do cross version support. Would you like to give that a shot?

if(CMAKE_VERSION VERSION_LESS "3.1")
include(CheckCXXCompilerFlag)
CHECK_CXX_COMPILER_FLAG("-std=c++11" COMPILER_SUPPORTS_CXX11)
if(NOT COMPILER_SUPPORTS_CXX11)
MESSAGE(FATAL_ERROR "Your compiler does not support c++11")
endif()
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
else()
set(CMAKE_CXX_STANDARD 11)
endif()

Similar to the discussion here, having cross version support in my opinion defeats the purpose of this PR. This PR is intended to use a cleaner method of choosing CXX standard, but if both are still present it only adds unnecessary complexity.

My advise would be to drop official support for Ubuntu 14.04 (it's over 3 years old ;) ) and provide a guide for installing cmake >3.1 if necessary. The alternative is to ignore this PR in favor of support on Ubuntu 14.04.

[hgaiser]

@Yangqing what is the verdict?

[Yangqing]

@hgaiser I asked around a little bit and seems that Cmake <3.0 is still something preferred... so maybe let's keep the old way? It's definitely not very ideal and I in fact recommend putting the backward-compatible, but newer approach for 3.1 :)

[williford]

@Yangqing I implemented your suggestion in PR #1027. Would you take a look when you get the chance?

[Yangqing]

Closing in favor of #1027, thanks so much guys :)