std::ios_base::pword

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

See also

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