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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.
They enable conditional linking, conditional definitions used when compiling, conditional include directories, and more. The conditions may be based on the build configuration, target properties, platform information or any other queryable information.
Generator expressions have the form $<...>. To avoid confusion, this page deviates from most of the CMake documentation in that it omits angular brackets <...> around placeholders like condition, string, target, among others.
Generator expressions can be nested, as shown in most of the examples below.
Boolean expressions evaluate to either 0 or 1. They are typically used to construct the condition in a conditional generator expression.
Available boolean expressions are:
Converts string to 0 or 1 according to the rules of the if() command. Evaluates to 0 if any of the following is true:
Otherwise evaluates to 1.
1 if string1 and string2 are equal, else 0. The comparison is case-sensitive. For a case-insensitive comparison, combine with a string transforming generator expression,
$<STREQUAL:$<UPPER_CASE:${foo}>,"BAR"> # "1" if ${foo} is any of "BAR", "Bar", "bar", ...
1 when the language used for compilation unit matches language, 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 string. For example,
include_directories(/usr/include/$<CXX_COMPILER_ID>/)
expands to /usr/include/GNU/ or /usr/include/Clang/ etc, depending on the compiler identifier.
String-valued expressions may also be combined with other expressions. Here an example for a string-valued expression within a boolean expressions within a conditional expression:
$<$<VERSION_LESS:$<CXX_COMPILER_VERSION>,4.2.0>:OLD_COMPILER>
expands to OLD_COMPILER if the CMAKE_CXX_COMPILER_VERSION is less than 4.2.0.
And here two nested string-valued expressions:
-I$<JOIN:$<TARGET_PROPERTY:INCLUDE_DIRECTORIES>, -I>
generates a string of the entries in the INCLUDE_DIRECTORIES target property with each entry preceded by -I.
Expanding on the previous example, if one first wants to check if the INCLUDE_DIRECTORIES property is non-empty, then it is advisable to introduce a helper variable to keep the code readable:
set(prop "$<TARGET_PROPERTY:INCLUDE_DIRECTORIES>") # helper variable
$<$<BOOL:${prop}>:-I$<JOIN:${prop}, -I>>
The following string-valued generator expressions are available:
String literals to escape the special meaning a character would otherwise have:
Conditional generator expressions depend on a boolean condition that must be 0 or 1.
Typically, the condition is a boolean generator expression. For instance,
$<$<CONFIG:Debug>:DEBUG_MODE>
expands to DEBUG_MODE when the Debug configuration is used, and otherwise expands to the empty string.
Content of expr 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>>
)
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.
Since generator expressions are evaluated during generation of the buildsystem, and not during processing of CMakeLists.txt files, it is not possible to inspect their result with the message() command.
One possible way to generate debug messages is to add a custom target,
add_custom_target(genexdebug COMMAND ${CMAKE_COMMAND} -E echo "$<...>")
The shell command make genexdebug (invoked after execution of cmake) would then print the result of $<...>.
Another way is to write debug messages to a file:
file(GENERATE OUTPUT filename CONTENT "$<...>")