On Tue, Jun 6, 2017 at 8:50 AM, Stephen Kelly <steve...@gmail.com> wrote:
> Craig Scott wrote: > > > On Tue, Jun 6, 2017 at 7:50 AM, Stephen Kelly > > <steve...@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? > Sorry, my comments were based on the example code which looks as though it was expecting C++11 to be a minimum requirement. The paragraph that follows it describes a different scenario, as you say. Apologies for the confusion! > > > 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. > Until very recently, that only worked for language features, it didn't help if you relied on parts of the STL, for example. I think the recently added cxx_std_?? compiler meta feature comes close to giving the behaviour I was describing now, but it is less convenient in that you always have to set it for every target, there isn't an associated variable that sets a default for all targets (that I'm aware of - I'd be happy to be wrong on this one!). Roger, to get back to your original question, Stephen's suggestion of not setting CMAKE_CXX_STANDARD_REQUIRED is probably going to give you what you are seeking. My view is that setting just CMAKE_CXX_STANDARD leads to unintuitive behaviour for many developers, but since you've explicitly stated that it is indeed the behaviour that you want, then I guess it works for you. Just be prepared to explain it to other developers from time to time!
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