std::ios_base::pword
| 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.
#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) |