Standard library header <cstdalign> (C++11)(until C++20), <stdalign.h> (C++11)

This header was originally in the C standard library as <stdalign.h>.

C compatibility header. alignas and alignof are keywords in C++, but they were not keywords in C until C23.

Macros
__alignas_is_defined (C++11)(deprecated)	C compatibility macro constant, expands to integer literal 1 (macro constant)

Notes

<cstdalign> is deprecated in C++17 and removed in C++20. Corresponding <stdalign.h> is still available in C++20.

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
LWG 3827	C++11	C programs no longer need the compatibility macro __alignas_is_defined since C23	deprecated this macro in C++