cmake: Don't forcibly export C++ version

[alexdewar]

This PR fixes issue #51, if the user has cmake v3.1 or newer, which is almost certainly most users.

For older versions of cmake there doesn't seem to be a nice way to check if the user has already specified a C++ version, so I have preserved the old behaviour where the C++ version is also set for any parent projects.

[ddemidov]

Travis reports a compile error:
https://travis-ci.org/ddemidov/ev3dev-lang-cpp/builds/524842540

[ddemidov]

Another option would be to set public target_compile_features on the library (example).

[alexdewar]

Rats. It looks like now the C++ version isn't being set to 11 when it should be.

The problem with using target_compile_features is that I'd like to be able to just use C++14 without having to specify which features of it I need.

I've tried tweaking it to see if this fixes things.

[alexdewar]

Ok, got it on the third attempt! It seems that Travis wasn't correctly setting the C++ version for gcc -- not sure why (it's a new version of cmake but an old version of gcc so maybe that's the problem).

So now we set the C++ version in two ways, which is a bit gross, but at least works and solves the original problem.

[ddemidov]

Are you sure it is safe to compile the library with c++11 and then link it with the code that was compiled with c++14? Are there no ABI incompatibilities?

[alexdewar]

I don't think there should be (C++ is supposed to be backwards compatible with this kind of thing...) and it does work for me.

Also it's quite a common thing to do: lots of C++ libs are compiled for C++11 but you can still link against them with programs using newer versions of C++.

[ddemidov]

I did a quick test in 11c2063, and it seems that just defining CMAKE_CXX_STANDARD earlier in CMakeLists.txt is enough:

https://travis-ci.org/ddemidov/ev3dev-lang-cpp/builds/524885989

So may be there is no need to test for CMAKE_VERSION?

[ddemidov]

Sorry, forgot to also remove -std=c++0x. Checking now:
https://travis-ci.org/ddemidov/ev3dev-lang-cpp/builds/524890788

[alexdewar]

That would be cleaner, but I believe CMAKE_CXX_STANDARD was only introduced in cmake v3.1. Could always just bump the cmake_minimum_required to 3.1 though, if you're sure that no one will be using an older version.

[ddemidov]

Looks like Debian stretch (the current distribution used by ev3dev) has cmake 3.7:

https://packages.debian.org/stretch/devel/cmake

so bumping the required version to 3.1 should be ok.

[alexdewar]

That all looks good to me. I've tested it with my project and now it works fine. Cheers :-)

[ddemidov]

Can you update this PR so I can merge to master?

[alexdewar]

Done :-)

[ddemidov]

Thank you!

[alexdewar]

Thanks for being so responsive.