CMake dependencies and Dll copies - c++

Question

I did wrote some CMake code to transitively get all library dependencies of a given executable target and copy dlls next to it (yes Windows World)
Then I got an external library that does not support CMake. That have a single implib lets call it a.lib with an a.dll, this library also depends on another dll without any available implib (b.dll)
So I wrote a aConfig.cmake like this:
set(a_FOUND TRUE)
add_library(a SHARED IMPORTED)
set_target_properties(a PROPERTIES
IMPORTED_LINK_INTERFACE_LANGUAGES "CXX"
IMPORTED_LOCATION "${a_DIR}/bin/a.dll"
IMPORTED_IMPLIB "${a_DIR}/lib/a.lib"
)
add_library(b SHARED IMPORTED)
set_target_properties(b PROPERTIES
IMPORTED_LOCATION "${a_DIR}/bin/b.dll"
)
# to express a dependency without requiring implib at link time
target_link_library(a PRIVATE b)
I end up with this (expected) error
IMPORTED library can only be used with the INTERFACE keyword of
target_link_libraries
Then I modified b to become an INTERFACE (It feels strange here):
add_library(b INTERFACE IMPORTED)
set_target_properties(b PROPERTIES
IMPORTED_LOCATION "${a_DIR}/bin/b.dll"
)
target_link_library(a INTERFACE b)
with CMake 3.18 my transitive loop does not want to ask for an IMPORTED_LOCATION on an INTERFACE
INTERFACE_LIBRARY targets may only have whitelisted properties. The
property "IMPORTED_LOCATION" is not allowed.
but with CMake 3.19 it works and do exactly what I am expecting. So I read the Release Note and except this statement :
Interface Libraries may now have source files added via add_library() or target_sources(). Those with sources will be generated as part of the build system.
nothing seems related to this.
Since I found the CMake 3.18 error, I feel that what I am doing is wrong but I cannot find a better way to express this runtime dependency.
Does anyone have a better idea on how to express this dependency or does it feels right like this ?

Answer 1

In CMake 3.19 and after you could create b target for a dll without a lib in a single line:
add_library(b INTERFACE "${a_DIR}/bin/b.dll")
So when you link a with b, a will get a dependency from the file ${a_DIR}/bin/b.dll.
This is what CMake 3.19 introduces: a dependency of INTERFACE library from the file.
For implement that dependency in CMake pre-3.19, one need to create additional custom target:
add_custom_target(b_files DEPENDS "${a_DIR}/bin/b.dll")
add_library(b INTERFACE)
add_dependencies(b b_files)