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 the ...
is true, else 0
$
1
if all ?
are 1
, else 0
The ?
must always be either 0
or 1
in boolean expressions.
$
0
if all ?
are 0
, else 1
$
0
if ?
is 1
, else 1
$
1
if a
is STREQUAL b
, else 0
$
1
if a
is EQUAL b
in a numeric comparison, else 0
$
1
if config is cfg
, else 0
. This is a case-insensitive comparison. The mapping in MAP_IMPORTED_CONFIG_<CONFIG>
is also considered by this expression when it is evaluated on a property on an IMPORTED
target.$
1
if the CMake-id of the platform matches comp
, otherwise 0
.$
1
if the CMake-id of the C compiler matches comp
, otherwise 0
.$
1
if the CMake-id of the CXX compiler matches comp
, otherwise 0
.$
1
if v1
is a version greater than v2
, else 0
.$
1
if v1
is a version less than v2
, else 0
.$
1
if v1
is the same version as v2
, else 0
.$
1
if the version of the C compiler matches ver
, otherwise 0
.$
1
if the version of the CXX compiler matches ver
, otherwise 0
.$
1
if the policy pol
was NEW when the ‘head’ target was created, else 0
. If the policy was not set, the warning message for the policy will be emitted. This generator expression only works for a subset of policies.$
1
if all of the feature
features are available for the ‘head’ target, and 0
otherwise. If this expression is used while evaluating the link implementation of a target and if any dependency transitively increases the required C_STANDARD
or CXX_STANDARD
for the ‘head’ target, an error is reported. See the cmake-compile-features(7)
manual for information on compile features and a list of supported compilers.$
1
when the language used for compilation unit matches lang
, otherwise 0
. This expression used to specify compile options for source files of a particular language in a target. For example, to specify the use of the -fno-exceptions
compile option (compiler id checks elided):
add_executable(myapp main.cpp foo.c bar.cpp) target_compile_options(myapp PRIVATE $<$<COMPILE_LANGUAGE:CXX>:-fno-exceptions> )
This generator expression has limited use because it is not possible to use it with the Visual Studio generators. Portable buildsystems would not use this expression, and would create separate libraries for each source file language instead:
add_library(myapp_c foo.c) add_library(myapp_cxx foo.c) target_compile_options(myapp_cxx PUBLIC -fno-exceptions) add_executable(myapp main.cpp) target_link_libraries(myapp myapp_c myapp_cxx)
The Makefile
and Ninja
based generators can also use this expression to specify compile-language specific compile definitions and include directories:
add_executable(myapp main.cpp foo.c bar.cpp) target_compile_definitions(myapp PRIVATE $<$<COMPILE_LANGUAGE:CXX>:COMPILING_CXX> ) target_include_directories(myapp PRIVATE $<$<COMPILE_LANGUAGE:CXX>:/opt/foo/cxx_headers> )
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:
$
CONFIG
instead.$
$
CMAKE_SYSTEM_NAME
variable.$
CMAKE_<LANG>_COMPILER_ID
variable.$
CMAKE_<LANG>_COMPILER_ID
variable.$
CMAKE_<LANG>_COMPILER_VERSION
variable.$
CMAKE_<LANG>_COMPILER_VERSION
variable.$
tgt
is the name of a target.$
$
$
tgt
is the name of a target.$
$
$
tgt
is the name of a target.$
$
$
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.
$
prop
on the target on which the generator expression is evaluated.$
install(EXPORT)
and empty otherwise.$
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:
$<0:...>
...
)$<1:...>
...
$
...
$
>
. Used to compare strings which contain a >
for example.$
,
. Used to compare strings which contain a ,
for example.$
;
. Used to prevent list expansion on an argument with ;
.$<...>
...
as being the name of a target. This is required if exporting targets to multiple dependent export sets. The ...
must be a literal name of a target- it may not contain generator expressions.$<...>
...
except when evaluated in a link interface while propagating Transitive Usage Requirements, in which case it is the empty string. Intended for use only in an INTERFACE_LINK_LIBRARIES
target property, perhaps via the target_link_libraries()
command, to specify private link dependencies without other usage requirements.$<...>
...
when the property is exported using install(EXPORT)
, and empty otherwise.$<...>
...
when the property is exported using export()
, or when the target is used by another target in the same buildsystem. Expands to the empty string otherwise.$<...>
...
converted to lower case.$<...>
...
converted to upper case.$<...>
...
converted to a C identifier.$
objLib
. objLib
must be an object of type OBJECT_LIBRARY
. This expression may only be used in the sources of add_library()
and add_executable()
commands.$<...>
...
converted to shell path style. For example, slashes are converted to backslashes in Windows shells and drive letters are converted to posix paths in MSYS shells. The ...
must be an absolute path.
© 2000–2016 Kitware, Inc.
© 2000–2011 Insight Software Consortium
Licensed under the BSD 3-clause License.
https://cmake.org/cmake/help/v3.6/manual/cmake-generator-expressions.7.html