cmake error：string no output variable specified

Execute the command: cmake ..

Or cmake.. -DCMAKE_EXPORT_COMPILE_COMMANDS=1, sometimes an error will be reported:

string no output variable specified 

This error is not a grammatical problem. It was caused by not specifying the default CMAKE_BUILD_TYPE when CMakeLists.txt was written, so you only need to specify the build type.

In the command line: cmake .. -DCMAKE_BUILD_TYPE=Debug -DCMAKE_EXPORT_COMPILE_COMMANDS=1

Or set the default CMAKE_BUILD_TYPE in the top CMakeLists.txt