[CMake] C++ standard version fallbacks.

[Stephen Kelly]

Craig Scott wrote:

> On Tue, Jun 6, 2017 at 7:50 AM, Stephen Kelly
> <steveire at gmail.com> wrote:
> 
>> Roger Leigh wrote:
>>
>> > Hi folks,
>> >
>> > I'm currently using this logic to use C++14 with a fallback to C++11
>> > when C++14 is unavailable:
>> >
>> >    if(NOT CMAKE_CXX_STANDARD)
>> >      set(CMAKE_CXX_STANDARD 14)
>> >    endif()
>> >    if(NOT CMAKE_CXX_STANDARD_REQUIRED)
>> >      set(CMAKE_CXX_STANDARD_REQUIRED 11)
>> >    endif()
>> >
>> > which seems to work OK.
>> >
>> > However, for some new stuff, I'd like to use C++17 when available, but
>> > fall back to C++14, C++11 or C++98.  Is it possible to do this?
>>
>> Probably set CMAKE_CXX_STANDARD
>>
>> without CMAKE_CXX_STANDARD_REQUIRED (That variable doesn't really make
>> sense
>> to me and I think it is overused when not needed).
>>
> 
> If you don't set CMAKE_CXX_STANDARD_REQUIRED, then there's no guarantee
> you get any particular minimum standard.

He wants to fall back all the way to C++98. Am I missing something?

> Roger's example (sorry Roger!)
> highlights part of the confusion about this latter variable (and the
> target property it ultimately controls). He appears to be setting it
> expecting it to specify a minimum version, but that's not how it works. It
> is expected to be a boolean which says whether CMAKE_CXX_STANDARD must be
> honoured or not, which most developers (myself included) tend to find
> unintuitive. 

Ok.

I remember I was opposed to introducing CMAKE_CXX_STANDARD_REQUIRED in the 
first place as I think it is redundant. I recommend populating compile 
features for whatever you absolutely need and let cmake populate the std 
flag. If your code can benefit from a more-recent std flag than the 
requirement, then set CMAKE_CXX_STANDARD to that.

> Roger's use would actually make it a bit better, if that was
> how it worked, but unfortunately there's currently no way to set a
> *minimum* standard version,

If you have a minimum, then you must be relying on some language features 
existing and you can list those.

Thanks,

Steve.