Dealing with lots and lots of escaped characters in add_custom_command - cmake

Question

I have a file that contains a bunch of data. I want to turn it into a C++ string literal, because I need to compile this data into the binary - I cannot read it from disk.
One way of doing this is to just generate a C++ source file that declares a string literal with a known name. The CMake code to do this is straightforward, if somewhat awful:
function(make_literal_from_file dest_file source_file literal_name)
add_custom_command(
OUTPUT ${dest_file}
COMMAND printf \'char const* ${literal_name} = R\"\#\(\' > ${dest_file}
COMMAND cat ${source_file} >> ${dest_file}
COMMAND printf \'\)\#\"\;\' >> ${dest_file}
DEPENDS ${source_file})
endfunction()
This works and does what I want (printf is necessary to avoid a new line after the raw string introducer). However, the amount of escaping going on here makes it very difficult to see what's going on. Is there a way to write this function such that it's actually readable?
Note that I cannot use a file(READ ...)/configure_file(...) combo here because source_file could be something that is generated by CMake at build time and so may not be present at configuration time.

Answer 1

I would recommend writing a script to do this. You could write it in CMake, but I personally prefer a better language such as Python:
# Untested, just to show roughly how to do it
import sys
dest_file, source_file, literal_name = sys.argv[1:]
with open(dest_file) as dest, open(source_file) as source:
literal_contents = source.read()
dest.write(f'char const* {literal_name} = R"({literal_contents})";\n')
Corresponding CMake code:
# String interpolation came in Python 3.6, thus the requirement on 3.6.
# If using CMake < 3.12, use find_package(PythonInterp) instead.
find_package(Python3 3.6 COMPONENTS Interpreter)
# Make sure this resolves correctly. ${CMAKE_CURRENT_LIST_DIR} is helpful;
# it's the directory containing the current file (this cmake file)
set(make_literal_from_file_script "path/to/make_literal_from_file.py")
function(make_literal_from_file dest_file source_file literal_name)
add_custom_command(
OUTPUT "${dest_file}"
COMMAND
"${Python3_EXECUTABLE}" "${make_literal_from_file_script}"
"${dest_file}"
"${source_file}"
"${literal_name}"
DEPENDS "${source_file}")
endfunction()
If you don't want the dependency on Python, you could use C++ (only the CMake code shown):
add_executable(make_literal_from_file_exe
path/to/cpp/file.cpp
)
function(make_literal_from_file dest_file source_file literal_name)
add_custom_command(
OUTPUT "${dest_file}"
COMMAND
make_literal_from_file_exe
"${dest_file}"
"${source_file}"
"${literal_name}"
DEPENDS "${source_file}")
endfunction()