econjack:
A valid pointer can only hold one of two values: 1) the lvalue of a variable of a specific data type, or 2) null.
I don't think that's quite accurate. What it means for a pointer to be 'valid' is open for debate, but if you mean that the pointer can be safely dereferenced then arguably null is not a valid value (unless you are actually accessing data at address zero), but the pointer is valid if it holds the address of any memory location where the pointed-to type can be accessed - it doesn't need to be the address of a variable. For example, if you are storing data in heap memory then it would be normal to use pointers to access that but there would be no corresponding variables at those locations.