CMake won't build without set_source_files_properties with LANGUAGE CXX - c++

Question

I have a small library that I have made (mostly wrappers for a more obtuse library underneath) which I have been compiling and using no problem in a contained project. I am now using this library in another project and have attempted to change the CMakeLists.txt appropriately (see below).
cmake_minimum_required (VERSION 3.5)
set(project "foobar")
project(${project} LANGUAGES CXX)
set(${project}_VERSION_MAJOR 0)
set(${project}_VERSION_MINOR 1)
add_library(${project} SHARED
./driver/foo.h
./driver/foo.c
./bar.cpp
./bar.hpp)
set_source_files_properties(./driver/foo.c PROPERTIES LANGUAGE CXX)
target_compile_features(${project}
PUBLIC
cxx_std_11)
target_include_directories(${project} PUBLIC ./driver/ .)
set_target_properties(${project} PROPERTIES LINKER_LANGUAGE CXX)
add_executable(bno055-test
./testingProject.cpp
)
target_link_libraries(test ${project})
install(
TARGETS ${project}
RUNTIME DESTINATION bin)
The error I have encountered is with the line set_source_files_properties(./driver/foo.c PROPERTIES LANGUAGE CXX). When it comes to compiling this C file with the C++ flag above I get many warnings of the type clang-8: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated [-Wdeprecated]. This is expected, as I am using deprecated methods and should not be compiling this file using the C++ flag. However, whenever I remove this line, or alter it to specify C instead of C++, my entire project no longer builds and fails with error:
CMake Error: Error required internal CMake variable not set, cmake may not be built correctly.
Missing variable is:
CMAKE_C_COMPILE_OBJECT
-- Generating done
CMake Generate step failed. Build files cannot be regenerated correctly.
Makefile:283: recipe for target 'cmake_check_build_system' failed
make: *** [cmake_check_build_system] Error 1
"/usr/bin/make -j4 all" terminated with exit code 2. Build might be incomplete.
I'm not familiar enough with CMake to see why this line of code and error are related like this. Is there some alternative piece of code I should include to create the missing variable CMAKE_C_COMPILE_OBJECT?
I assume that when I initially created this project using Eclipse this line was automatically generated and I am unclear as to why it now creates a problem when I had been compiling these files with the same CMakeLists.txt before (without library linking).
This is with clang#8.0.0 on Ubuntu 16.04.
Any help would be much appreciated! Apologies if I have missed something simple!

Answer 1

Is there some alternative piece of code I should include to create the missing variable CMAKE_C_COMPILE_OBJECT?
No, as the CMake error states, this is an "internal CMake variable" which should be initialized by CMake. It is not something you should have to set yourself. It should be set indirectly when you call project(). However, you only tell CMake you are using C++ (with CXX option) in the project() command. If you want CMake to make use of C and C++, you need to add both:
project(${project} LANGUAGES C CXX)
Even better, CMake enables C and C++ by default, so you can simply do:
project(${project})