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.

#include <ciso646>
#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
#error Using an unknown standard library

Possible output:

main.cpp:7:2: error: Using Microsoft STL
#error Using Microsoft STL
1 error generated.

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