how does the ampersand(&) sign work in c++?

Question

Possible Duplicate:
What are the differences between pointer variable and reference variable in C++?

This is confusing me:

class CDummy 
{
public:
   int isitme (CDummy& param);
};

int CDummy::isitme (CDummy& param)
{
  if (&param == this)
  { 
       return true; //ampersand sign on left side??
  }
  else 
  {    
       return false;
  }
}

int main () 
{
  CDummy a;
  CDummy* b = &a;

  if ( b->isitme(a) )
  {
    cout << "yes, &a is b";
  }

  return 0;
}

In C & usually means the address of a var. What does it mean here? Is this a fancy way of pointer notation?

The reason I am assuming it is a pointer notation because this is a pointer after all and we are checking for equality of two pointers.

I am studying from cplusplus.com and they have this example.

Answer 1

To start, note that

this

is a special pointer ( == memory address) to the class its in. First, an object is instantiated:

CDummy a;

Next, a pointer is instantiated:

CDummy *b;

Next, the memory address of a is assigned to the pointer b:

b = &a;

Next, the method CDummy::isitme(CDummy &param) is called:

b->isitme(a);

A test is evaluated inside this method:

if (&param == this) // do something

Here's the tricky part. param is an object of type CDummy, but &param is the memory address of param. So the memory address of param is tested against another memory address called "this". If you copy the memory address of the object this method is called from into the argument of this method, this will result in true.

This kind of evaluation is usually done when overloading the copy constructor

MyClass& MyClass::operator=(const MyClass &other) {
    // if a programmer tries to copy the same object into itself, protect
    // from this behavior via this route
    if (&other == this) return *this;
    else {
        // otherwise truly copy other into this
    }
}

Also note the usage of *this, where this is being dereferenced. That is, instead of returning the memory address, return the object located at that memory address.

Answer 2

The & has more the one meanings:

1) take the address of a variable

int x;
void* p = &x;
//p will now point to x, as &x is the address of x

2) pass an argument by reference to a function

void foo(CDummy& x);
//you pass x by reference
//if you modify x inside the function, the change will be applied to the original variable
//a copy is not created for x, the original one is used
//this is preffered for passing large objects
//to prevent changes, pass by const reference:
void fooconst(const CDummy& x);

3) declare a reference variable

int k = 0;
int& r = k;
//r is a reference to k
r = 3;
assert( k == 3 );

4) bitwise and operator

int a = 3 & 1; // a = 1

n) others???

Answer 3

Well the CDummy& param that is declared as a parameter of the function CDummy::isitme is actually a reference which is "like" a pointer, but different. The important thing to note about references is that inside functions where they are passed as parameters, you really have a reference to the instance of the type, not "just" a pointer to it. So, on the line with the comment, the '&' is functioning just like in C, it is getting the address of the argument passed in, and comparing it to this which is, of course, a pointer to the instance of the class the method is being called on.