# std::ios_base::xalloc

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< cpp‎ | io‎ | ios base

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:

    cout, narrow-character test
    myout, narrow-character test [special handling for mystream]
    wcout, wide-character test
    mywout, wide-character test [special handling for mystream]

### See also

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

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