Sorry, I made an error last time, I corrected this :
replaced "%module Str" by "%module Str" into the str.i file.

Now the require instruction works fine, but the type "error" is still 
the same :

#--------------------#
[gnargeot@dptinfo ex3]$ swig -c++ -ruby *.i
[gnargeot@dptinfo ex3]$ gcc -c -fpic *_wrap.cxx -I. -I$RBDIR
[gnargeot@dptinfo ex3]$ gcc -shared -fpic *_wrap.o -lstdc++ -o Str.so
[gnargeot@dptinfo ex3]$ irb --simple-prompt
 >> require 'Str.so'
=> true
 >> include Str
=> Object
 >> bidon
=> #<SWIG::TYPE_p_string:0xb7efa238>
#--------------------#

Best regards,

Guillaume