Currently the code coverage of the https://github.com/BlueBrain/nmodl/blob/master/src/codegen/codegen_cpp_visitor.cpp responsible for printing almost all of the C++ code is around ~89% according to the code coverage report you can find attached.
nmodl_coverage.zip
The code coverage was generated using the following script https://github.com/BlueBrain/nmodl/blob/magkanar/codecov/codecov.sh.
This issue is about trying to increase it as much as possible to 100% with the addition of unit or integration tests.