void *& pword(int idx);The member function returns a reference to element idx of the extensible array with elements of type void pointer. All
elements are effectively present and initially store the null pointer. The returned reference is invalid after the next call to
pword for the object, after the object is altered by a call to basic_ios::copyfmt, or after the object is destroyed.
To obtain a unique index for use across all objects of type ios_base, call xalloc.