std::ios_base::xalloc
From cppreference.com
static int xalloc();
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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.
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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.
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]
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
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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) |
