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 std::basic_ios::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 the function fails (possibly caused by an allocation failure) and *this is a base
class subobject of a basic_ios<> object or subobject, 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]

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
the stored value might not be the stored value is retained
LWG 36 C++98 retained if the reference is until the next call of
invalidated copyfmt()
the function set badbit by itself on badbit is set by basic_ios
LWG 41 C++98 failure, (if *this is its base class
but ios_base does not provide such subobject)
interface

See also¶

resizes the private storage if necessary and access to the long element at
iword the given index
(public member function)
xalloc returns a program-wide unique integer that is safe to use as index to
[static] pword() and iword()
(public static member function)