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Generator expressions are evaluated during build system generation to produce information specific to each build configuration.
Generator expressions are allowed in the context of many target properties, such as LINK_LIBRARIES, INCLUDE_DIRECTORIES, COMPILE_DEFINITIONS and others. They may also be used when using commands to populate those properties, such as target_link_libraries(), target_include_directories(), target_compile_definitions() and others.
This means that they enable conditional linking, conditional definitions used when compiling, and conditional include directories and more. The conditions may be based on the build configuration, target properties, platform information or any other queryable information.
Logical expressions are used to create conditional output. The basic expressions are the 0 and 1 expressions. Because other logical expressions evaluate to either 0 or 1, they can be composed to create conditional output:
$<$<CONFIG:Debug>:DEBUG_MODE>
expands to DEBUG_MODE when the Debug configuration is used, and otherwise expands to nothing.
Available logical expressions are:
1 if all ? are 1, else 0
The ? must always be either 0 or 1 in boolean expressions.
1 when the language used for compilation unit matches lang, otherwise 0. This expression may be used to specify compile options, compile definitions, and include directories for source files of a particular language in a target. For example:
add_executable(myapp main.cpp foo.c bar.cpp zot.cu)
target_compile_options(myapp
PRIVATE $<$<COMPILE_LANGUAGE:CXX>:-fno-exceptions>
)
target_compile_definitions(myapp
PRIVATE $<$<COMPILE_LANGUAGE:CXX>:COMPILING_CXX>
$<$<COMPILE_LANGUAGE:CUDA>:COMPILING_CUDA>
)
target_include_directories(myapp
PRIVATE $<$<COMPILE_LANGUAGE:CXX>:/opt/foo/cxx_headers>
)
This specifies the use of the -fno-exceptions compile option, COMPILING_CXX compile definition, and cxx_headers include directory for C++ only (compiler id checks elided). It also specifies a COMPILING_CUDA compile definition for CUDA.
Note that with Visual Studio Generators and Xcode there is no way to represent target-wide compile definitions or include directories separately for C and CXX languages. Also, with Visual Studio Generators there is no way to represent target-wide flags separately for C and CXX languages. Under these generators, expressions for both C and C++ sources will be evaluated using CXX if there are any C++ sources and otherwise using C. A workaround is to create separate libraries for each source file language instead:
add_library(myapp_c foo.c)
add_library(myapp_cxx bar.cpp)
target_compile_options(myapp_cxx PUBLIC -fno-exceptions)
add_executable(myapp main.cpp)
target_link_libraries(myapp myapp_c myapp_cxx)
These expressions expand to some information. The information may be used directly, eg:
include_directories(/usr/include/$<CXX_COMPILER_ID>/)
expands to /usr/include/GNU/ or /usr/include/Clang/ etc, depending on the Id of the compiler.
These expressions may also may be combined with logical expressions:
$<$<VERSION_LESS:$<CXX_COMPILER_VERSION>,4.2.0>:OLD_COMPILER>
expands to OLD_COMPILER if the CMAKE_CXX_COMPILER_VERSION is less than 4.2.0.
Available informational expressions are:
Full path to the linker generated program database file (.pdb) where tgt is the name of a target.
See also the PDB_NAME and PDB_OUTPUT_DIRECTORY target properties and their configuration specific variants PDB_NAME_<CONFIG> and PDB_OUTPUT_DIRECTORY_<CONFIG>.
Value of the property prop on the target tgt.
Note that tgt is not added as a dependency of the target this expression is evaluated on.
These expressions generate output, in some cases depending on an input. These expressions may be combined with other expressions for information or logical comparison:
-I$<JOIN:$<TARGET_PROPERTY:INCLUDE_DIRECTORIES>, -I>
generates a string of the entries in the INCLUDE_DIRECTORIES target property with each entry preceded by -I. Note that a more-complete use in this situation would require first checking if the INCLUDE_DIRECTORIES property is non-empty:
$<$<BOOL:${prop}>:-I$<JOIN:${prop}, -I>>
where ${prop} refers to a helper variable:
set(prop "$<TARGET_PROPERTY:INCLUDE_DIRECTORIES>")
Available output expressions are:
Content of ... evaluated as a generator expression in the context of tgt target. This enables consumption of custom target properties that themselves contain generator expressions.
Having the capability to evaluate generator expressions is very useful when you want to manage custom properties supporting generator expressions. For example:
add_library(foo ...)
set_property(TARGET foo PROPERTY
CUSTOM_KEYS $<$<CONFIG:DEBUG>:FOO_EXTRA_THINGS>
)
add_custom_target(printFooKeys
COMMAND ${CMAKE_COMMAND} -E echo $<TARGET_PROPERTY:foo,CUSTOM_KEYS>
)
This naive implementation of the printFooKeys custom command is wrong because CUSTOM_KEYS target property is not evaluated and the content is passed as is (i.e. $<$<CONFIG:DEBUG>:FOO_EXTRA_THINGS>).
To have the expected result (i.e. FOO_EXTRA_THINGS if config is Debug), it is required to evaluate the output of $<TARGET_PROPERTY:foo,CUSTOM_KEYS>:
add_custom_target(printFooKeys
COMMAND ${CMAKE_COMMAND} -E
echo $<TARGET_GENEX_EVAL:foo,$<TARGET_PROPERTY:foo,CUSTOM_KEYS>>
)