赋值运算符c ++ NULL赋值

Question

I am working with a class that is mostly working fine, but I am doing some functionality that I might be having a recursive function return a NULL pointer of the class type as a control, and so it might be assigning a NULL pointer to my class object so long story short:

Thing& Thing::operator=(Thing * _other){
    if (_other == NULL){
        *this = NULL;        // compiler throws here
        return *this;    
    }
    // does other assignment work
    return *this;
}

my compiler VS2010 throws that this is not an I-value. so how do I set the value to NULL , or is it even possible to set an item to NULL from inside?

EDIT: modified this to *this though for some reason now the program breaks with infinite calls to the assignment operator. no idea what is going on

Answer 1

You cannot directly set a value to the this pointer. Hence

this = NULL;

is semantically and syntactically wrong.

You can use exceptions to check if _other is NULL . For example:

class null_object_exception : public virtual exception {};
...
if (_other == NULL) throw null_object_exception();

To perform a NULL assignment:

Thing the_thing, other_thing;
try { the_thing = &other_thing; }
catch( const null_object_exception& e ) { the_thing = NULL; }

Answer 2

You are trying to write a "nullable" class. Consider the "null" state to be one of the states an instance of Thing can be in, and don't convolute it with pointer semantics.

The general method would be to add a boolean flag to your class that keeps track of whether an instance is in null state or not. I would implement it like this:

class Thing
{
private:
    bool m_null;

public:
    void setnull() { m_null = true; }
    bool isnull() const { return m_null; }

    Thing() : m_null(true) {}

    ... // rest of class
};

And now the default assignment operator works fine.

Answer 3

You CANNOT assign to this .

Also, you should take your arguments by const reference, so Thing& operator= (const Thing& other)

There's a great SO question in C++-faq tag about operator overloading, you can find it here

Answer 4

what are the member variables of Thing class? if you want to show that the object is somehow without value or not intialized, you would better assign fileds 0 (for integers) and null(for pointers) etc, instead of assigning "this" which is constant.

Answer 5

Short answer, no, you can't assign to this in C++.

Longer answer; for your assignment operator to even be called, you'd have to have a construct like;

MyObject a;
a = NULL;    // can change the content of `a` but not which object the name `a` refers to.

If you're thinking about this construct;

MyObject *a;
a = NULL;

your assignment operator won't even be called since it is an operator on the object , not the pointer . Assigning to the pointer a will work without you defining an assignment operator.