std::ios_base::xalloc

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< cpp‎ | io‎ | ios base
 
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std::ios_base
 
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++14)

Effectively increments a private static data member of std::ios_base, as if by executing return index++;, if index is the name of that static member (which may be std::atomic to support concurrent access by multiple threads, or otherwise synchronized) (since C++14)

Parameters

(none)

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]

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)
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