13.15 <cstddef>

The <cstddef> header is the C++ version of the C standard <stddef.h> header, which declares a few types and macros.

The C header declares the wchar_t type, but wchar_t is a reserved keyword in C++, so there is no need to #include <cstddef> to declare this type.

#define NULL  . . .

The NULL macro expands to a null pointer constant, such as 0 or 0L.

Some C libraries declare NULL as ((void*)0) in stddef.h. This definition is fine for C, but is wrong for C++. Most C++ compilers correctly declare NULL, but you should be aware of the difference.

size_t offsetof(type, member-name)

The offsetof macro returns the offset, in bytes, of a member of a struct as a constant integer. The type must be a plain, C-style struct (Plain Old Data, or POD), and the expression &(t.member-name) must be an address constant, assuming t is an instance of type. In particular, this means the member-name must not be a bit-field, a static member, or a function member. (See Chapter 6 for more information about POD types.)

typedef ... ptrdiff_t

The ptrdiff_t type is a signed integral type that represents the difference between two pointers. The exact type is implementation-defined.

typedef ... size_t

The size_t type is the type of the result of the sizeof operator. It is an unsigned integral type. The exact type is implementation-defined.