[CMake] Can an option enforce a default, even if cache is present?

[Petr Kmoch]

On Tue, 27 Nov 2018 at 21:42, frodak <frodak17 at gmail.com> wrote:

> On Tue, Nov 27, 2018 at 3:15 PM Mario Emmenlauer <mario at emmenlauer.de>
> wrote:
> >
> > On 27.11.18 17:13, Eric Noulard wrote:
> > > Le mar. 27 nov. 2018 à 14:50, Mario Emmenlauer <mario at emmenlauer.de
> > > <mailto:mario at emmenlauer.de>> a écrit :
> > >     Dear all,
> > >
> > >     I've just discovered that option() behaves differently than I
> anticipated.
> > >     After reading the docs and searching with google I'm still
> confused how to
> > >     achieve my desired behaviour.
> > >
> > >     What I've just learned is that unspecified options take their
> cached value
> > >     and do *not* go back to their default value, if a cache exists. I
> assumed
> > >     that options take their default when not explicitly specified.
> > >
> > >
> > > The behavior of option() gained new behavior in CMake 3.13.
> > > May be it could help in your particular case:
> > > https://cmake.org/cmake/help/v3.13/policy/CMP0077.html#policy:CMP0077
> > >
> > > you'll depend on latest CMake though.
> > >
> > >
> > >     Now my problem: I could not find a way to get the behaviour I'd
> like. Is it
> > >     possible to enforce the default for an option when its not
> specified by the
> > >     user, even if a cache exists?
> > >
> > >
> > > You mean you did not manage to force the cache value?
> > > You can:
> > > set(VAR "default_value" CACHE FORCE)
> > >
> > > or the problem is you cannot know whether if a value has been
> user-provided?
> >
> > Sorry, I was not very precise! Your last point is the problem. I fail to
> know
> > when the option was user-provided and when it was cache-provided.
> >
> > So here is what I'd like:
> >
> > #> grep MYOPT CMakeLists.txt
> > option(MYOPT "Description" OFF)
> > #> cmake              # I want the option disabled, this works fine.
> > #> cmake -DMYOPT=ON   # I want the option enabled, this works fine.
> > #> cmake              # I want the option disabled (back to default),
> >                        # but I observe the option taken from cache,
> enabled.
> >
> > Is there some way to achieve my desired behaviour? I tried without
> success
> > unset(MYOPT), unset(MYOPT CACHE), and set(MYOPT OFF) before option(), but
> > they all lead to different behaviour.
> >
>

You're forgetting one important aspect of CMake: that it can retrigger
itself when a CMake source file changes. Such a run of CMake is
indistinguishable from running it manually, and it's (a large part of) why
the cache exists in the first place. Imagine the following scenario:

User runs >cmake -DMYOPT=ON
User edits a CMakeLists.txt (or even just a file processed by
configure_file()).
User runs >make

As part of this make, CMake triggers to regenerate the buildsystem. If
MYOPT exhibited the behaviour you request, it would silently get disabled
again, even though this is most definitely not what the user expects. I
addressed a similar point in this StackOverflow answer:
https://stackoverflow.com/a/41361741/1782465

Petr


>
> I've always used 'cmake -UMYOPT'  to remove specific items from the
> cache (or edit the cache file directly).
> I don't think the notion of user-provided or cache-provided entries
> exist because all user defined variables go into the cache.
> Then you can test to see if MYOPT is set and then use the default
> value when recreating the cache entry.
> Also cmake cache variables are persistent between invocations so the
> user doesn't need to keep specifying them at the command line every
> time cmake needs to run.
>
> Best regards...
> --
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