std::ios_base::xalloc(3)

NAME
std::ios_base::xalloc - std::ios_base::xalloc

Synopsis
   static int xalloc();
   Returns a unique (program-wide) index value that can be used to access one long and
   one void* elements in the private storage of std::ios_base by calling iword() and
   pword(). The call to xalloc does not allocate memory.
   This function is thread-safe: concurrent access by multiple threads (since C++11)
   does not result in a data race.
   Effectively increments the next available unique index.

Return value
   Unique integer for use as pword/iword index.

Example
   Uses base class pword storage for runtime type identification of derived stream
   objects. // Run this code
 #include <iostream>
 template<class CharT, class Traits = std::char_traits<CharT>>
 class mystream : public std::basic_ostream<CharT, Traits>
 {
 public:
     static const int xindex;
     mystream(std::basic_ostream<CharT, Traits>& ostr) :
         std::basic_ostream<CharT, Traits>(ostr.rdbuf())
     {
         this->pword(xindex) = this;
     }
     void myfn()
     {
         *this << "[special handling for mystream]";
     }
 };
 // Each specialization of mystream obtains a unique index from xalloc()
 template<class CharT, class Traits>
 const int mystream<CharT, Traits>::xindex = std::ios_base::xalloc();
 // This I/O manipulator will be able to recognize ostreams that are mystreams
 // by looking up the pointer stored in pword
 template<class CharT, class Traits>
 std::basic_ostream<CharT, Traits>& mymanip(std::basic_ostream<CharT, Traits>& os)
 {
     if (os.pword(mystream<CharT, Traits>::xindex) == &os)
         static_cast<mystream<CharT, Traits>&>(os).myfn();
     return os;
 }
 int main()
 {
     std::cout << "cout, narrow-character test " << mymanip << '\n';
     mystream<char> myout(std::cout);
     myout << "myout, narrow-character test " << mymanip << '\n';
     std::wcout << "wcout, wide-character test " << mymanip << '\n';
     mystream<wchar_t> mywout(std::wcout);
     mywout << "mywout, wide-character test " << mymanip << '\n';
 }

Output:
 cout, narrow-character test
 myout, narrow-character test [special handling for mystream]
 wcout, wide-character test
 mywout, wide-character test [special handling for mystream]
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 2143 C++11 xalloc was not thread-safe made thread-safe

See also
         resizes the private storage if necessary and access to the void* element at
   pword the given index
         (public member function)
         resizes the private storage if necessary and access to the long element at the
   iword given index
         (public member function)