Re: Checking assignment operators with *this pointer

"Jim Langston" <>
Tue, 24 Apr 2007 20:38:01 -0700

"john" <> wrote in message

 I know we use the pointer this to obtain a class object or class
member data. I don't follow the reason for example this code. I'am
quite confused

assingment operator
const B &operator=(const B& x){

          if (&x != this)
          a = x.a;

          return *this;

This is how i see this code. its compares the reference of x to the
adress of the this pointer??

In a line like this, i don't follow the steps the compiler takes.
b1 = b2;

Code below

[snip code]

It's to check for self assignment. Consider.

myclass A;

A = A;

This could also occur using pointers. You get an instance of myclass using
new and through the course of the program wind up trying to assign the
instance to itself.

In a trivial program where a is a built in type it's not a problem. But
there are casses where you may be storing things on the stack. Consider the
case where a is actually a pointer. An assignment operator would be tempted
to do something like this:

// consider a to be an int pointer. consider size to be a variable with
it's size.
const B &operator=(const B& x)
     delete a;
     a = new int[x.size]
     for ( size_t i = ; i < size; ++i )
        a[i] = x.a[i];

      return *this;

Seems all well and good. We delete our current array, create a new one with
the proper size, then copy the contents. But what happens when self
assignment occurs? When x is actually this. As soon as we delete a we just
screwed up. We no longer have the memory to copy from, since we just
released it. That's where we check to make sure we aren't doing self

const B &operator=(const B& x)
     if ( &x != this )
         delete a;
         a = new int[x.size]
         for ( size_t i = ; i < size; ++i )
            a[i] = x.a[i];

      return *this;

this is a pointer to our current instance. It compares it with the address
of the instance to copy from. If they are the same, then self assignment is
occuring. We don't have to copy or assign anything, since we'd just be
copying from ourselves.

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