std::ios_base::iword - cppreference.com

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# std::ios_base::iword

From cppreference.com
< cpp‎ | io‎ | ios base

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 xalloc(), otherwise collisions with other users of this `ios_base` may occur. New elements are initialized to ​0​.

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

### [edit] 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.

### [edit] Parameters

index | - | index value of the element

### [edit] Return value

A reference to the element.

### [edit] Exceptions

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

### [edit] 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

### [edit] See also

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

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