“取消引用空指针”究竟是什么意思?

NULL 0x00000000 *

int a, b, c; // some integers
int *pi;     // a pointer to an integer

a = 5;
pi = &a; // pi points to a
b = *pi; // b is now 5
pi = NULL;
c = *pi; // this is a NULL pointer dereference

NullReferenceException

* x *x & &x & y

*(&x) == x
&(*y) == y

int *x = NULL;  // x is a null pointer
int y = *x;     // CRASH: dereference x, trying to read it
*x = 0;         // CRASH: dereference x, trying to write it

NullReferenceException NullPointerException

myclass *p = NULL;
*p = ...;  // illegal: dereferencing NULL pointer
... = *p;  // illegal: dereferencing NULL pointer
p->meth(); // illegal: equivalent to (*p).meth(), which is dereferencing NULL pointer

myclass *p = /* some legal, non-NULL pointer */;
*p = ...;  // Ok
... = *p;  // Ok
p->meth(); // Ok, if myclass::meth() exists

(*p) p->... (*p). ...

wiki

int val =1;
int *p = NULL;
*p = val; // Whooosh!!!! 

wikipedia

作为 取消引用 指针。

取消引用是通过应用 unary * 指针上的运算符。

int x = 5;
int * p;      // pointer declaration
p = &x;       // pointer assignment
*p = 7;       // pointer dereferencing, example 1
int y = *p;   // pointer dereferencing, example 2

“取消对空指针的引用”表示执行 *p 当 p 是 NULL