Is a NULL pointer's dereference also equals NULL?

问题

Does the following snippet compile/execute the block in the if-statement?

int* pointer = NULL;
int deref = *pointer;
if(deref == NULL){
  // will execute?
}

Since the pointer variable contains NULL does the dereference of this pointer variable also return NULL or will this result in a runtime error?

回答1:

The result is "undefined behaviour", which may or may not trigger a runtime error, and should in any case always be avoided.

回答2:

Once you set:

int* pointer = NULL;

pointer points to nothing. Now when you write this:

int deref = *pointer;

deref will try to access what pointer points to, which will lead to an undefined behaviour like segmentation fault.

I hope this explains.

回答3:

The current answers addressed the UB very well. However, I want to add something. If you run this code:

if(0==NULL){
    std::cout << "True";
}

It will prints True. So if dereferencing a null pointer on your specific environment leads to returning 0 (which is not a steady case. It is UB), the part inside your if statement will execute.

I just wanted to clarify why it is working on some machines. However, that does not change anything about the fact that it is UB.

来源：https://stackoverflow.com/questions/39608081/is-a-null-pointers-dereference-also-equals-null