Build

$ cd ext/bar
$ ruby extconf.rb
$ make

Test result

$ ruby -Iext -rbar/bar -e 'f=[]; 100000.times {f<<"foo"}; b=Bar.new; p b.bar_good, b.bar_bad;'
"bar"
"foo"

Note:

bar_bad is initialized as "bar"; however it prints "foo";
bar_bad probably causes segmentation fault instead of printing "foo".

Tested ruby versions:

ruby 2.3.1p112 (2016-04-26 revision 54768) [x86_64-linux]
ruby 2.4.0p0 (2016-12-24 revision 57164) [x86_64-linux]