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Showing content from https://en.cppreference.com/w/cpp/language/../error/error_code/../../io/ios_base/xalloc.html below:

std::ios_base::xalloc - cppreference.com

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.

[edit] Return value

Unique integer for use as pword/iword index.

[edit] Example

Uses base class pword storage for runtime type identification of derived stream objects.

#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]
[edit] 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 [edit] See also resizes the private storage if necessary and access to the void* element at the given index
(public member function) [edit] resizes the private storage if necessary and access to the long element at the given index
(public member function) [edit]

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