Reference pointing to a Null-object

问题:

I saw this discussion - Checking for a null object in C++ and I was surprised that no one talked about when reference can point to a null object. In our code, we use null objects routinely. There are functions as follows which return nullObj.

const Obj&  
nullObj()  
{  
   static obj* nullPtr = NULL;   
   return static_cast<  const Obj&>(*nullPtr);    
}  

Actually, when I looked at the code again to bring this topic up, I had some questions on how the above code works:

1. How is it possible to do *nullPtr - Is is it because nullPtr is a static object, which is allocated memory on the heap and hence it is guaranteed to have some space and

2. Since we are returning const reference to obj, does compiler create a temporary object (to some kind of nullObj??) or Will the const reference act as an alias to nullPtr itself?

回答1:

I was surprised that no one talked about when reference can point to a null object

That's because it can't, in a correct program.

Your function that dereferences a nullpointer has Undefined Behavior. The compiler is allowed to emit code that, for example, causes a crash at that point.

However, one possible effect of UB is that the code does what one thought it would do. So null-references can occur. I have never encountered one, but if you do, then it means that there is a serious logic error in the code.

All uses of the null-object-reference function you show, are logic errors.

You'd better grep up those uses and fix things. ;-)

Cheers & hth.,

回答2:

I was surprised that no one talked about when reference can point to a null object.

Never.

(i) How is it possible to do *nullPtr - Is is it because nullPtr is a static object, which is allocated memory on the heap and hence it is guarenteed to have some space and address allocated for deref?

It's not. You're dereferencing a null pointer, which is invoking undefined behaviour.

(ii) Since we are returning const reference to obj, does compiler create a temporary object (to some kind of nullObj??) or Will the const reference act as an alias to nullPtr itself?

No. The compiler, at this stage, is allowed to produce nasal daemons or a black hole. If you're very lucky, you'll get a segmentation fault or some other kind of access violation.

DO NOT DO THIS

回答3:

(i) How is it possible to do *nullPtr - Is is it because nullPtr is a static object, which is allocated memory on the heap and hence it is guarenteed to have some space and address allocated for deref?

No, it's because nullPtr is a pointer initialized to NULL, it's not an object at all.

(ii) Since we are returning const reference to obj, does compiler create a temporary object (to some kind of nullObj??) or Will the const reference act as an alias to nullPtr itself?

It will be an alias to an object located at memory address NULL. I.e.: when you actually access any of the members of this reference object - you'll get whatever (usually access violation, if the system is smart enough, but it can really be anything, the behavior is undefind).