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std::ios_base::xalloc(3) C++ Standard Libary std::ios_base::xalloc(3) 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 ac- cess one long and one void* elements in the private storage of std::ios_base by call- ing iword() and pword(). The call to xalloc does not allocate memory. This function is thread-safe: concurrent access by multiple threads (since C++11) does not result in a data race. 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 de- rived 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 xal- loc() 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 retroac- tively 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 resizes the private storage if necessary and access to the void* element at pword the given index (public member function) resizes the private storage if necessary and access to the long element at the iword given index (public member function) http://cppreference.com 2024.06.10 std::ios_base::xalloc(3)
NAME | Synopsis | Return value | Example | Output: | See also
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