[CMake] Re: Re: FindQt4 in 2.4.8 bug

[Bill Hoffman]

Hendrik Sattler wrote:
> Quoting Bill Hoffman <bill.hoffman at kitware.com>:
>> OK, I get it now...   So, the FIND_* stuff will set a value if it is
>> NOTFOUND.  However, the set command will not.  I am thinking it should.
>>     I am not sure what that will break, but it would be consistent with
>> FIND_*.
>>
>> This should not be a problem if you start from a clean build tree for
>> FindQt4.cmake, as QT_INCLUDE_DIR will never get put in the cache as
>> NOTFOUND by the current FindQt4.cmake.   So, I think this is something
>> we can fix in CMake 2.6.    Feel free to create a feature request, "set
>> CACHE should set NOTFOUND variables".
> 
> Let's say a library is optional. A current script cannot see the 
> difference anymore between "library not found on earlier run" and "I did 
> not search for it, yet". SET is well-documented about CACHE and FORCE 
> behaviour. Don't break that as you can always use
>  if (NOT DEFINED VAR)
>    set (VAR "foo" CACHE ...... FORCE)
> 

But:

find_library(FOO_VAR foo)

This will set FOO_VAR (in the cache) if it is not set, or if the value 
has -NOTFOUND in it.

However,

set(FOO_VAR somevalue CACHE STRING "")

Will only set FOO_VAR if it has no value at all.  Seems a bit 
inconsistent.

-Bill