[CMake] What does find_package(COMPONENTS) do? and

[Stephen Kelly]

Hi,

The documentation says

> A package-specific list of components may be listed after the REQUIRED 
> option or after the COMPONENTS option if no REQUIRED option is given. 

http://www.cmake.org/cmake/help/cmake-2-8-docs.html#command:find_package

But it doesn't say why you would want to do that. At first I thought it was 
to specify that only the components specified would be used to fill cmake 
variables. However, find_package(Qt COMPONENTS QtCore) also results in 
${QT_QTGUI_LIBRARY} being defined for example. So then I thought maybe the 
semantic is that if one of the components can not be found, the find_package 
fails. So I tried find_package(Qt REQUIRED QtCore QtDoesNotExist) which 
works without error.

What am I missing?

The reason I ask is that I finally figured out why the tests in my library 
needed to be linked to QtGui, even though it only uses QtCore.

The reason is that find_package(Qt) causes QT_GUI_LIB to be defined. Which 
in turn causes QTEST_MAIN to be defined to expand to use QApplication 
instead of QCoreApplication as it is if QT_GUI_LIB is not defined.

http://qt.gitorious.org/qt/qt/blobs/4.7/src/testlib/qtest.h

find_package(Qt REQUIRED QtCore QtScript) does not cause QT_GUI_LIB to be 
defined. However, some of my targets do need QtGui, so I should specify it, 
right? 

So if I do specify it I'll end up having QT_GUI_LIB defined when building my 
unit tests. I could remove_definitions(-DQT_GUI_LIB), but apart from being a 
BadHack(tm), it causes my unit tests which *do* require QtGui to fail at 
runtime because they create QWidgets and by undeffing QT_GUI_LIB I build 
them to use QCoreApplication instead of QApplication (Not allowed in Qt), so 
the tests fail at runtime.

http://gitorious.org/grantlee/grantlee/blobs/0.1/tests/CMakeLists.txt

Is there a solution to all this? What is the point of COMPONENTS if it has 
no effect on what I can include or link to? Is it possible to link some of 
my tests to QtGui but not all of them and still have them all pass? Do I 
need to just link my core tests to QtGui and use QApplication and quit my 
complaining?

The only way I can see to satisfy all requirements is attached. Is that 
acceptable or is there a better way?

All the best,

Steve.
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