It is used to get new index for extensible array [static] and returns a new index value to be used with member functions in the internal extensible array.
The internal extensible array is a general-purpose array of objects of type long (if accessed with member iword) or void* (if accessed with member pword).
Following is the declaration for ios_base::xalloc function.
static int xalloc();
A new index that can be used with either member iword or member pword.
Strong guarantee − if an exception is thrown, there are no changes in any stream.
Concurrently calling this function may introduce data races.