Expand variable name containing a generator expression in cmake - cmake

Question

In the build process, I set directories where I gather the build output of different sub-projects. The directories are set as :
set( CMAKE_RUNTIME_OUTPUT_DIRECTORY_DEBUG "${CMAKE_CURRENT_LIST_DIR}/../build/bin/debug" )
set( CMAKE_RUNTIME_OUTPUT_DIRECTORY_RELEASE "${CMAKE_CURRENT_LIST_DIR}/../build/bin/release" )
Now, I'd like to copy some files (a directory of qt plugins) to that directory dependent on the configuration which it is built for.
I tried:
# copy qt plugins
add_custom_command( TARGET mytarget POST_BUILD
COMMAND ${CMAKE_COMMAND} -E copy_directory
"${QT_DIR}/../../../plugins"
"${$<UPPER_CASE:CMAKE_RUNTIME_OUTPUT_DIRECTORY_$<CONFIG> >}/plugins"
COMMAND_EXPAND_LISTS)
thus, I try to build a string that equals the variable name and then try to expand that as described here: CMake interpret string as variable. In other words: I would like to have a generator expression that evaluates to the content of CMAKE_RUNTIME_OUTPUT_DIRECTORY_DEBUG or CMAKE_RUNTIME_OUTPUT_DIRECTOR_RELEASE dependent on the current build configuration.
However running cmake with the statement above results in an error:
"CMakeLists.txt:112: error: Syntax error in cmake code at [..] when parsing string ${$<UPPER_CASE:CMAKE_RUNTIME_OUTPUT_DIRECTORY_$<CONFIG> >}/plugins Invalid character ('<') in a variable name: '$'
So my question is, how can I use a generator-expression to access the corresponding variable? (Bonus question: is there another/better way to achieve the same goal?)

Answer 1

So my question is, how can I use a generator-expression to access the corresponding variable?
You cannot. There is currently (CMake <=3.23) no way to expand a variable whose name is determined by the value of a generator expression.
Bonus question: is there another/better way to achieve the same goal?
Yes, and you are almost there! You can use $<TARGET_FILE_DIR:...>:
add_custom_command(
TARGET mytarget POST_BUILD
COMMAND
${CMAKE_COMMAND} -E copy_directory
"${QT_DIR}/../../../plugins"
"$<TARGET_FILE_DIR:mytarget>/plugins"
VERBATIM
)
This works because TARGET_FILE_DIR evaluates to the actual directory containing the executable or library file for mytarget, no matter the active configuration, property values, etc.
Docs: https://cmake.org/cmake/help/latest/manual/cmake-generator-expressions.7.html#genex:TARGET_FILE_DIR
CMAKE_RUNTIME_OUTPUT_DIRECTORY_<CONFIG> is already relative to the binary directory so you should not try to compute the binary directory in its definition. Also, it supports generator expressions. Thus, the following will be much more robust:
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY "bin/$<LOWER_CASE:$<CONFIG>>"
CACHE STRING "Common output directory for runtime artifacts")
This has a bunch of concrete benefits:
No need to set CMAKE_RUNTIME_OUTPUT_DIRECTORY_DEBUG or CMAKE_RUNTIME_OUTPUT_DIRECTORY_RELEASE
This will work for MinSizeRel and RelWithDebInfo, plus any custom configurations one might add down the line.
Since it's defined as a cache variable, it can be overridden for debugging / working around name clashes, etc.
A bit more context for (3): most CMAKE_* variables are intended to be either read-only or user-configurable (i.e. at the command line, from the GUI, etc.). Overriding their defaults via set(CACHE) is a polite compromise. A notable exception to this rule is the collection of Qt codegen flags (CMAKE_AUTO{MOC,RCC,UIC}). These must typically be set for the build to produce usable binaries.