std::ios_base::xalloc
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ios_base::xalloc
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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.
[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.
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]
[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]
(public member function) [edit]