std::ios_base:: xalloc
|
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.
#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
|
resizes the private storage if necessary and access to the
void
*
element at the given index
(public member function) |
|
|
resizes the private storage if necessary and access to the
long
element at the given index
(public member function) |