NAME

std::ios_base::pword - std::ios_base::pword

Synopsis

void*& pword( int index );

First, allocates or resizes the private storage (dynamic array of void* or another
indexable data structure) sufficiently to make index a valid index, then returns a
reference to the void* element of the private storage with the index index.

The reference may be invalidated by any operation on this ios_base object, including
another call to pword(), but the stored values are retained, so that reading from
pword(index) with the same index later will produce the same value (until the next
call to copyfmt()). The value can be used for any purpose. The index of the element
must be obtained by xalloc(), otherwise the behavior is undefined. New elements are
initialized to a null pointer.

If allocation fails, calls std::basic_ios<>::setstate(badbit) which may throw
std::ios_base::failure

Parameters

index - index value of the element

Return value

A reference to the element.

Exceptions

May throw std::ios_base::failure when setting the badbit.

Notes

If the pointers stored in pword require management, register_callback() may be used
to install handlers that execute deep copy or deallocation as needed.

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]