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 does not result
in a data race.
(since C++14)

Effectively increments a private static data member of std::ios_base, as if by
executing return index++;, if index is the name of that static member
(which may be std::atomic to support concurrent access by multiple threads, or
otherwise synchronized)
(since C++14)

Parameters

(none)

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: