Line 105: usage();
should be followed by "exit();"
Otherwise, compilation fails on Ubuntu 18.04 with message:
fasta_formatter.cpp:105:9: error: this statement may fall through [-Werror=implicit-fallthrough=]
usage();
~~~~~^~
fasta_formatter.cpp:107:3: note: here
case 'i':
^~~~
cc1plus: all warnings being treated as errors