[cmake-developers] Target usage requirements and conventions

[Alexander Neundorf]

On Sunday 29 April 2012, Stephen Kelly wrote:
> Hi there,
> 
> The topic of 'target usage requirements' has come up several times. It's
> something I'd like to work further towards in CMake 2.8.9.
> 
> I have created a wiki page on the KDE wiki (there for my convenience
> mostly) so that we can discuss the design, implementation and
> implications.
> 
> http://community.kde.org/Frameworks/Epics/CMake_target_usage_requirements
> 
> Thanks for any comments,

Some comments:

You propose target_use_package(Foo Bar) to state that the target Foo should 
use the include dirs and libraries from Bar. I like the idea of having a 
separate command for that.


I do not like the idea that target_use_package() should automatically do a 
find_package() for several reasons.
* find_package() has many options, in the end they would all have to be 
present there too
* find_package() is an important part of the CMakeLists.txt, it should not be 
hidden
* wouldn't this fall back to a find_package() call with its associated error 
messages at cmake-time for every typo in the package name ?


Looking at other cmake commands, many of them now have two modes, an old one, 
where simply the options are listed without separating keywords:
find_library(FOO_LIBRARY foo /opt/foo /usr/foo/lib)
and a new, more powerful one with additional keywords:
find_library(FOO_LIBRARY NAMES foo PATHS /opt/foo /usr/foo/lib)

When adding a new command, we should start with the keyword-based variant, to 
avoid having two signatures in the future.
This also makes the code more explicit about what it does, less guessing 
involved for readers:

target_use_package(Foo IMPORTED_TARGETS Bar Blub)
or
target_use_package(Foo VIA_PREFIX Bar_Bat )
or
target_use_package(Foo PACKAGE Bar)


Doing it this way would also solve the problem that your prefix-based example:

target_use_package(Foo ${Bar_Bat_LIBRARIES})
"... Otherwise, the variables with the appropriate naming conventions (eg 
Bar_Bat_LIBRARIES Bar_Bat_INCLUDE_DIRS) are used."

the target_use_package() implementation would get to see that value of 
${Bar_Bat_LIBRARIES}, which might be "/usr/lib/libbat.so;/usr/lib/libbar.so", 
and wouldn't be able to find out about prefixes anymore.

Alex