The use of namespace std for all identifiers of th C++ standard library was introduced during the standardization process. So saying that namespace std holds all the class shipped with the compiler is correct.Backward compatible is also provided by including C header files. (example #include )
ios class is derived from ios_base class which has opening mode flag eg. in,out etc.
These stream opening mode flag are static const public member
of ios_base class
and can be directly accessed by calling their names as ios_base::in
or by the class derived from ios_base class. ios
class is derived from ios_base
so stream opening flag can be accessed as ios::in
.