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# Useful Modules
There are a ton of useful modules in CMake's «cmake:modules» collection; but some of them are more useful than others. Here are a few highlights.
## «module:CMakeDependentOption»
This adds a command `cmake_dependent_option` that sets an option based on another set of variables being true. It looks like this:
```cmake
include(CMakeDependentOption)
cmake_dependent_option(BUILD_TESTS "Build your tests" ON "VAL1;VAL2" OFF)
```
which is just a shortcut for this:
```cmake
if(VAL1 AND VAL2)
set(BUILD_TESTS_DEFAULT ON)
else()
set(BUILD_TESTS_DEFAULT OFF)
endif()
option(BUILD_TESTS "Build your tests" ${BUILD_TESTS_DEFAULT})
if(NOT BUILD_TESTS_DEFAULT)
mark_as_advanced(BUILD_TESTS)
endif()
```
Note that `BUILD_TESTING` is a better way to check for testing being enabled if you use `include(CTest)`, since it is defined for you. This is just an example of `CMakeDependentOption`.
## «module:CMakePrintHelpers»
This module has a couple of handy output functions. `cmake_print_properties` lets you easily print properties.
And `cmake_print_variables` will print the names and values of any variables you give it.
## «module:CheckCXXCompilerFlag»
This checks to see if a flag is supported. For example:
```cmake
include(CheckCXXCompilerFlag)
check_cxx_compiler_flag(-someflag OUTPUT_VARIABLE)
```
Note that `OUTPUT_VARIABLE` will also appear in the configuration printout, so choose a good name.
This is just one of many similar modules, such as `CheckIncludeFileCXX`, `CheckStructHasMember`, `TestBigEndian`, and `CheckTypeSize` that allow you
to check for information about the system (and you can communicate that to your source code).
## «command:`try_compile`»/«command:`try_run`»
This is not exactly a module, but is crucial to many of the modules listed above. You can attempt to compile (and possibly run) a bit of code at configure time. This can allow you to get information about the capabilities of your system. The basic syntax is:
```cmake
try_compile(
RESULT_VAR
bindir
SOURCES
source.cpp
)
```
There are lots of options you can add, like `COMPILE_DEFINITIONS`. In CMake 3.8+, this will honor the CMake C/C++/CUDA standard settings. If you use `try_run` instead, it will run the resulting program and give you the output in `RUN_OUTPUT_VARIABLE`.
## «module:FeatureSummary»
This is a fairly useful but rather odd module. It allows you to print out a list of packages what were searched for, as well as any options you explicitly mark. It's partially but not completely tied into «command:`find_package`». You first include the module, as always:
```cmake
include(FeatureSummary)
```
Then, for any find packages you have run or will run, you can extend the default information:
```cmake
set_package_properties(OpenMP PROPERTIES
URL "http://www.openmp.org"
DESCRIPTION "Parallel compiler directives"
PURPOSE "This is what it does in my package")
```
You can also set the `TYPE` of a package to `RUNTIME`, `OPTIONAL`, `RECOMMENDED`, or `REQUIRED`; you can't, however, lower the type of a package; if you have already added a `REQUIRED` package through «command:`find_package`» based on an option, you'll see it listed as `REQUIRED`.
And, you can mark any options as part of the feature summary. If you choose the same name as a package, the two interact with each other.
```cmake
add_feature_info(WITH_OPENMP OpenMP_CXX_FOUND "OpenMP (Thread safe FCNs only)")
```
Then, you can print out the summary of features, either to the screen or a log file:
```cmake
if(CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME)
feature_summary(WHAT ENABLED_FEATURES DISABLED_FEATURES PACKAGES_FOUND)
feature_summary(FILENAME ${CMAKE_CURRENT_BINARY_DIR}/features.log WHAT ALL)
endif()
```
You can build any collection of `WHAT` items that you like, or just use `ALL`.