Please validate the followings:
STL is thread safe at class level.
Maybe, it depends on the implementation of the standard library you are
using.
Iostream library is thread safe at object level.
Maybe, it depends on the implementation of the standard library you are
using.
what does it mean and how is it useful for the programmers.
The C++ standard does not say anything about multithreading issues. A
compliant implementation may ignore multithreading issues altogether
moving the burden of proper synchronisation to the user. OTOH there is
nothing in the standard that prevent library implementaors to add thread
synchronisation to their library. Either way you will have to consult
the documentation that came with the standard library you are using or
contact its vendor.