# 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++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<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

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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