[CMake] out of source command line flag

[Eric Noulard]

Continued....

2) In the same way I did file a bug report for being able to enforce
    out-of-source if necessary.
    http://public.kitware.com/Bug/view.php?id=6672

2009/9/5 Bill Hoffman <bill.hoffman at kitware.com>:

>
> I use this all the time, and so do lots of people that I know. They do this:

Yes right I do that too.

However it may be easy to follow Roman initial idea and replace:

cmake -B ./debug -DDebug=1 .

cmake -DDebug=1 ./debug .

i.e keep current usage:
cmake [options] <path-to-source>
cmake [options] <path-to-existing-build>

and add:
cmake [options] <path-to-build> <path-to-source>
cmake [options] <path-to-source> <path-to-build>

if you call cmake with 2 paths arguments then
one argument should be the source tree
other argument should be the build tree.
Note however that in this case <path-to-build> may not exist at all.

CMake may guess which one is the build tree using this algorithm.

1) If there is a CMakeCache.txt in the dir then this is a build tree
2) If the path does not exists then this is a "to be created" build tree

error cases are:

E1) One gives two existing dir with none containing a CMakeCache.txt
      ==> send an error message explaining that and abort.

E2) One gives two directories
          E21) both contains a CMakeCache.txt
      or E22) one contains a CMakeCache.txt and the other does not exists.

          E21 is plain wrong
          E22 the source tree may have been polluted with a previous
in-source config.

E3) One gives a build tree which does not correspond to the given source tree
         This is easy to check because CMakeCache.txt contains
                 <PROJECT>_SOURCE_DIR

      If this appear CMake should abort and send a message telling
      the source tree/build tree mismatch.

This does not look like that complicated and will keep full backward
compatibilty without adding extra "-B" option.

To be clear I'm not wanting to apply for providing a patch but may
be Roman would like to try ?

May be worth trying if other find the idea interesting.

-- 
Erk
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