First, allocates or resizes the private storage (dynamic array oflong or another indexable data structure) sufficiently to makeindex a valid index, then returns a reference to thelong element of the private storage with the indexindex.

The reference may be invalidated by any operation on thisios_base object, including another call toiword(), but the stored values are retained, so that reading fromiword(index) with the same index later will produce the same value until the next call tostd::basic_ios::copyfmt(). The value can be used for any purpose. The index of the element must be obtained by a previous call toxalloc(), 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 abasic_ios<> object or subobject, callsstd::basic_ios<>::setstate(badbit) which may throwstd::ios_base::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-definedoperator<< andoperator>> or to user-defined formatting facets imbued into standard streams.

[edit]Parameters

[edit]Return value

A reference to the element.

[edit]Exceptions

May throwstd::ios_base::failure when setting the badbit.

[edit]Example

#include <iostream>#include <string> struct Foo{staticint foo_xalloc;std::string data;      Foo(conststd::string& s): data(s){}}; // Allocates the iword storage for use with Foo objectsint Foo::foo_xalloc= std::ios_base::xalloc(); // This user-defined operator<< prints the string in reverse if the iword holds 1std::ostream& operator<<(std::ostream& os, Foo& f){if(os.iword(Foo::foo_xalloc)==1)return os<<std::string(f.data.rbegin(), f.data.rend());elsereturn os<< f.data;} // This I/O manipulator flips the number stored in iword between 0 and 1std::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';}

exampleelpmaxeexample

[edit]Defect reports

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

[edit]See also

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