From cppreference.com
staticintxalloc();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
and
. The call to xalloc does not allocate memory.
This function is thread-safe: concurrent access by multiple threads does not result in a data race.
(since C++11)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<classCharT,classTraits=std::char_traits<CharT>>classmystream:publicstd::basic_ostream<CharT,Traits>{public:staticconstintxindex;mystream(std::basic_ostream<CharT,Traits>&ostr):std::basic_ostream<CharT,Traits>(ostr.rdbuf()){this->pword(xindex)=this;}voidmyfn(){*this<<"[special handling for mystream]";}};// Each specialization of mystream obtains a unique index from xalloc()template<classCharT,classTraits>constintmystream<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 pwordtemplate<classCharT,classTraits>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();returnos;}intmain(){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
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 the given index
(public member function)
resizes the private storage if necessary and access to the long element at the given index
(public member function)