Standard library header <cstddef>
From cppreference.com
                    
                                        
                    
                    
                                                            
                    This header was originally in the C standard library as <stddef.h>.
This header is part of the utility library.
[edit] Macros
| implementation-defined null pointer constant (macro constant) | |
| byte offset from the beginning of a standard-layout type to specified member (function macro) | 
[edit] Types
| unsigned integer type returned by the sizeof operator (typedef) | |
| signed integer type returned when subtracting two pointers (typedef) | |
| (C++11) | the type of the null pointer literal nullptr (typedef) | 
| (C++11) | POD type with alignment requirement as great as any other scalar type (typedef) |