please send me some examoles of implementing str stream.
i am trying to understand it but i could not find proper source in the net.

what does str() do??

great stuff to implement. do you understand the cin, stringstream, check to see the functionality they provide, and what they basically are ( HINT: they are specialization of templates ).
if you are good C++ developer then you'll figure out what they exactly do.

typedef basic_stringstream<char, char_traits<char>, allocator<char> > stringstream;

The implement yours.
or if you just want to write it and you don't know about the traits and templates then make the one using buffers.