The <ciso646> header (from the C standard <iso646.h> header) does nothing in C++. The C header defines a small number of macros, such as and for &&, but these macros are all reserved keywords in C++. The <ciso646> header exists only for the sake of completeness: every header in the C standard has an equivalent in C++.
Not all C++ compilers correctly implement the alternative tokens such as and. These compilers might use <ciso646> to declare these keywords as macros. For maximum portability to these nonconforming compilers, include the <ciso646> header when you want to use any alternative token keywords. See Chapter 1 for more information about the alternative tokens.