为什么我在声明中说int * p = NULL，但在测试中p！= NULL，为什么不用* p！= NULL来匹配声明？

Question

in c++, if we assign a pointer value to NULL ,why dont we check if *p!=NULL and instead p!=NULL ?

I found this code in a tutorial.

int *p = NULL;
char *q = NULL;
// ...
if (p!=NULL) cout << *p;

Thanks in advance

Answer 1

The * is doing two different things. When you declare the variable, it means the variable is a pointer. When you use the variable, it means "dereference", that is take the value at the location the pointer is pointing to. Two totally different meanings.

Answer 2

因为p是指针，而*p是它指向的对象。

Answer 3

I think you are confused. The code from the tutorial is creating a pointer and initializing it to NULL , then later it is testing if it is NULL . The reason why you don't check if *p != NULL is because that would be testing if what it points to is NULL , and not test if the pointer itself is NULL .

Of course you can choose to test *p against any value you like provided it is a valid pointer... It all depends on what you want to do.

EDIT:

You didn't assign NULL to *p , you assigned it to p . The statement int *p = NULL; is the same as writing:

int *p;
p = NULL;

int * is the type.

Basically, when you write: if(p != NULL) you are simply testing if p points to a place such that it is safe to use *p .

Answer 4

The code is declaring a int pointer through the statement int *p = NULL . Please note that these statements are equivalent:

int *p = NULL;
int* p = NULL;

And both of these mean that p is a pointer to an integer which further means that p holds the address of an integer. So, when the code later checks for

if(p != NULL)

it is basically checking that whether the address contained in this pointer is NULL or not. I hope it makes things clear.

Answer 5

Because p is a pointer; it is a variable that holds a memory address of some object.

The code checks if the pointer is pointing to an object and, if it is, shows the value of that object on the screen.

---

if (p != NULL) // "if p is pointing to an object then ..."
{
    cout << *p; // "show me the value of that object on the screen"
}

• p is the address of the object
• *p is the value of the object, called dereferencing