NAME¶

std::ios_base::iword - std::ios_base::iword

Synopsis¶

long& iword( int index );

First, allocates or resizes the private storage (dynamic array of long or another
indexable data structure) sufficiently to make index a valid index, then returns a
reference to the long 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 iword(), but the stored values are retained, so that reading from
iword(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 a previous call to xalloc(), otherwise the
behavior is undefined. New elements are initialized to 0.

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.

Notes¶

Typical use of iword storage is to pass information (e.g. custom formatting flags)
from user-defined I/O manipulators to user-defined operator<< and operator>> or to
user-defined formatting facets imbued into standard streams.

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.

Example¶

// Run this code

#include <iostream>
#include <string>

struct Foo
{
static int foo_xalloc;
std::string data;

Foo(const std::string& s) : data(s) {}
};

// Allocates the iword storage for use with Foo objects
int Foo::foo_xalloc = std::ios_base::xalloc();

// This user-defined operator<< prints the string in reverse if the iword holds 1
std::ostream& operator<<(std::ostream& os, Foo& f)
{
if (os.iword(Foo::foo_xalloc) == 1)
return os << std::string(f.data.rbegin(), f.data.rend());
else
return os << f.data;
}

// This I/O manipulator flips the number stored in iword between 0 and 1
std::ios_base& rev(std::ios_base& os)
{
os.iword(Foo::foo_xalloc) = !os.iword(Foo::foo_xalloc);
return os;
}

int main()
{
Foo f("example");
std::cout << f << '\n' << rev << f << '\n' << rev << f << '\n';
}

Output:¶

example
elpmaxe
example

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 void* element at
pword 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)