Standard library header <ciso646> (until C++20), <iso646.h>
This header was originally in the C standard library as <iso646.h>.
Compatibility header, in C defines alternative operator representations which are keywords in C++.
This means that in a conforming implementation, including this header has no effect.
In old or nonconforming compilers, using the alternative operator representations may still require including this header.
<ciso646> is removed in C++20. Corresponding
<iso646.h> is still available in C++20.
Prior to C++20, including
<ciso646> was sometimes used as a technique for obtaining definitions of implementation-specific library version macros without causing other effects.
As of C++20, the header <version> was added for this purpose.
Run this code
#include <ciso646> #ifdef _LIBCPP_VERSION #error Using LLVM libc++ #elif __GLIBCXX__ // Note: only version 6.1 or newer define this in ciso646 #error Using GNU libstdc++ #elif _CPPLIB_VER // Note: used by Visual Studio #error Using Microsoft STL #else #error Using an unknown standard library #endif
main.cpp:7:2: error: Using Microsoft STL #error Using Microsoft STL ^ 1 error generated.