of course it builded before, I didn't add the files to CmakeLists. Now build fails

CMakeLists.txt

@@ -11,6 +11,7 @@ set(CMAKE_CXX_STANDARD_REQUIRED ON)
 # Core library
 add_library(core
     src/core/SysfsRead.cxx
+    src/core/Producer.cxx
 )
 
 target_include_directories(core PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}/include)

tests/CMakeLists.txt

@@ -12,4 +12,16 @@ target_link_libraries(test_sysfs_reader
         gtest_main
 )
 add_test(NAME test_sysfs_reader COMMAND test_sysfs_reader)
+add_executable(test_producer
+    test_producer.cxx
+)
+
+target_link_libraries(test_producer
+    PRIVATE
+        core
+        gtest
+        gtest_main
+)
+
+add_test(NAME test_producer COMMAND test_producer)