CMake define new target by string processing existing $<TARGET_FILE> - cmake

Question

I"m trying to modernize an older CMake script that essentially does the following to generate a libtool file:
get_target_property(target_location ${target} LOCATION)
get_filename_component(target_we ${target_location} NAME_WE)
get_target_property(target_deps ${target} LT_DEPENDENCY_LIBS)
# ...
# Get a bunch more properties...
# ...
set(la_target ${PROJECT_BINARY_DIR}/${target_we}.la)
# ...
# Do a bunch of file(WRITE...) file(APPEND...) etc
# ...
install(FILES ${la_target} DESTINATION ${CMAKE_INSTALL_PREFIX}/lib)
You cannot get the LOCATION property from a target in modern cmake, and I can't figure out how to do get_filename_component() on a $ generator.
Using the generator in an add_custom_command() COMMAND statement only allows one line, so I can't set variables to do all the necessary get_target_property()/file(WRITE...) processing.
Alternatively I can add a COMMAND cmake -P script.cmake to run a script which can do all the string processing but can't define or reference targets, so I appear to be stuck.
EDIT: I'm using the Makefile generator, on macOS, the target in question is a shared library that's generated in the same project.
EDIT2: The error is:
The LOCATION property may not be read from target "mytarget".
Use the target name directly with add_custom_command, or use
the generator expression $<TARGET_FILE>, as appropriate.

Answer 1

You can to it like this:
first use file command to expand generator expressions and render a level-2 template
... which is going to be rendered at build time by configure_file
Here is a sample project:
cmake_minimum_required(VERSION 3.10)
project(location-test VERSION 0.0.1 LANGUAGES CXX)
add_executable(foo foo.cc)
file(GENERATE OUTPUT blah.la.in INPUT blah.la.in.in)
configure_file(render-variables.cmake.in render-variables.cmake #ONLY)
add_custom_command(
OUTPUT "${PROJECT_BINARY_DIR}/blah.la"
COMMAND "${CMAKE_COMMAND}"
-DPROJECT_BINARY_DIR="${PROJECT_BINARY_DIR}"
-DPROJECT_NAME="${PROJECT_NAME}"
-DPROJECT_VERSION="${PROJECT_VERSION}"
-P "${PROJECT_BINARY_DIR}/render-variables.cmake"
MAIN_DEPENDENCY "${PROJECT_BINARY_DIR}/blah.la.in"
DEPENDS "${PROJECT_BINARY_DIR}/render-variables.cmake"
)
add_custom_target(
make-la-la-la ALL
DEPENDS "${PROJECT_BINARY_DIR}/blah.la"
)
blah.la.in.in (2-levels of tempaltes %):
# This file going to be rendered by the following pipeline:
# - at CMake execution time it'll expand all generator expressions
# (e.g. $<TARGET_FILE:foo>)
# - at build time, the `render-variables.cmake` script will substitute
# CMake variables (like #PROJECT_NAME# or #PROJECT_VERSION#)
write the template for your real .la file... render-variables.cmake.in is trivial as :
configure_file("${PROJECT_BINARY_DIR}/blah.la.in" "${PROJECT_BINARY_DIR}/blah.la" #ONLY)
And finally a dummy C++ file foo.cc:
#include <iostream>
int main()
{
std::cout << "Hello Africa!\n";
}
Build it!
localtion-sample$ mkdir build/
localtion-sample$ cd build/
localtion-sample/build$ cmake ..
-- The CXX compiler identification is GNU 5.4.0
-- Check for working CXX compiler: /usr/x86_64-pc-linux-gnu/gcc-bin/5.4.0/c++ -- works
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /tmp/localtion-sample/build
localtion-sample/build$ make
Scanning dependencies of target foo
[ 33%] Building CXX object CMakeFiles/foo.dir/foo.cc.o
[ 66%] Linking CXX executable foo
[ 66%] Built target foo
Scanning dependencies of target make-la-la-la
[100%] Generating blah.la
[100%] Built target make-la-la-la
localtion-sample/build$ ll
total 52K
drwxr-xr-x 6 zaufi zaufi 320 Feb 9 22:35 CMakeFiles/
-rw-r--r-- 1 zaufi zaufi 287 Feb 9 22:35 blah.la
-rw-r--r-- 1 zaufi zaufi 300 Feb 9 22:35 blah.la.in
-rw-r--r-- 1 zaufi zaufi 11K Feb 9 22:35 CMakeCache.txt
-rw-r--r-- 1 zaufi zaufi 1.5K Feb 9 22:35 cmake_install.cmake
-rwxr-xr-x 1 zaufi zaufi 14K Feb 9 22:35 foo
-rw-r--r-- 1 zaufi zaufi 5.0K Feb 9 22:35 Makefile
-rw-r--r-- 1 zaufi zaufi 89 Feb 9 22:35 render-variables.cmake
localtion-sample/build$ cat blah.la
# This file going to be rendered by the following pipeline:
# - at CMake execution time it'll expand all generator expressions
# (e.g. /tmp/localtion-sample/build/foo)
# - at build time, the `render-variables.cmake` script will substitute
# CMake variables (like location-test or 0.0.1)