说明：  sizeof操作符的作用是返回一个对象或类型名的长度，长度的单位是字节。返回值的类型是标准库命名为size_t的类型，size_t类型定义在cstddef头文件中，该头文件是C标准库的头文件stddef.h的C++版本。他是一个和机器相关的unsigned类型，其大小足以保证内存中对象的大小。
(The role of the sizeof operator returns an object or type name length, length in bytes. Return type is the standard type library named size_t, size_t types are defined in cstddef header file, the header file is C standard library of C header files stddef.h version. He is one and Machine-dependent unsigned type, its size is sufficient to ensure that the size of objects in memory.)