[cmake-developers] unset and cache variables

[Robert Maynard]

>  I can never actually make it become undefined

You can, you are missing unset(BLAH CACHE) to tell CMake you want to
undefine the CACHE varibles

> I had been operating under the assumption that there was only one active lookup scope

Nope CMake lookup is local scope, and than cache. That is how CMake
supports CACHE and local variables with the same name. The rules of
variable lookup are outlined at
https://cmake.org/cmake/help/latest/manual/cmake-language.7.html#variables
On Thu, Aug 16, 2018 at 6:00 PM James Touton <bekenn at gmail.com> wrote:
>
> Fair enough, but I think that makes the command substantially less useful.  It means that I need to use set(BLAH "") if I want to empty a variable of its contents, and I can never actually make it become undefined, counter to the claim in the documentation:
>
> set(BLAH blah CACHE INTERNAL "")
> unset(BLAH)
> if(DEFINED BLAH)
>     message("here") # this message gets printed!
> endif()
>
> I am now effectively subject to sabotage from above.
>
> I had been operating under the assumption that there was only one active lookup scope, constructed from the previous lookup scope and any new sets/unsets at the current level.  I therefore expected that if a variable were removed from "the" current scope, it would be inaccessible.
>
> On Thu, Aug 16, 2018 at 2:46 PM, Robert Maynard <robert.maynard at kitware.com> wrote:
>>
>> Looking at the current docs (
>> https://cmake.org/cmake/help/v3.12/command/unset.html ) I think the
>> first paragraph needs to be read as a single statement to understand
>> the behavior.
>>
>> "Removes the specified variable causing it to become undefined. If
>> CACHE is present then the variable is removed from the cache instead
>> of the current scope."
>>
>> I think we can be more explicit by rephrasing as:
>>
>> "Makes the local variable variable from the current scope be
>> undefined, If CACHE is present then instead the cache variable is
>> undefined."
>> On Thu, Aug 16, 2018 at 5:38 PM James Touton <bekenn at gmail.com> wrote:
>> >
>> > I just recently came across this:
>> >
>> > set(BLAH blah CACHE INTERNAL "")
>> > unset(BLAH)
>> > message("BLAH: ${BLAH}")
>> >
>> > Surprisingly (to me), this prints out "blah".  I had expected the unset command to make the cached value inaccessible.  The documentation just states that the function "[r]emoves the specified variable causing it to become undefined."  Is this a bug?  I would prefer for the function to guarantee that ${BLAH} comes up empty.
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