Defined in header
typedef /*implementation-defined*/ max_align_t;
max_align_t is a type whose alignment requirement is at least as strict (as large) as that of every scalar type.
Pointers returned by allocation functions such as malloc are suitably aligned for any object, which means they are aligned at least as strict as max_align_t.
max_align_t is usually synonymous with the largest scalar type, which is long double on most platforms, and its alignment requirement is either 8 or 16.
C++ documentation for max_align_t