指针（pointer）有两种涵义
一是指C/C++中的一种数据类型（data type）。
二是指某个类型为指针的 数据/物件（object）。
指针作为一种数据类型，属所谓复合类型（compound types）。
若非特别指明，下文中所言的指针皆为第二种涵义。
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
A compound type is a type that is defined in terms of another type. A pointer is a compound type that "points to" another type. Pointers are used for indirect access of other objects. A pointer is an object in its own right. A pointer holds the address of another object.
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Pointer Values
The value (i.e., the address) stored in a pointer can be in one of four states:
1. It can point to an object
2. It can point to the location just immediately past the end of an object
3. It can be a null pointer, indicating that it is not bound to any object
4. It can be invalid; values other than the preceeding three are invalid
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------