std::ios_base::xalloc - cppreference.com (original) (raw)

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.

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   template<class CharT, class Traits = std::char_traits> 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 myout(std::cout); myout << "myout, narrow-character test " << mymanip << '\n';   std::wcout << "wcout, wide-character test " << mymanip << '\n';   mystream 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.

[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] |