Assignment operator overloading

Hello,

I am having a hard time understanding the assignment operator overloading in a class that has a pointer as its member attribute. My question is: why does it make a memory leak if you do not delete the pointer before reassigning the value of the right hand side object? For example, there is a class called Number that has a pointer, itsNum, to an integer that is dynamically allocated (just as an example). Here is the code for assignment operator:

Code:

Number& Number::operator=(const Number& rhs)
{
    if (this != &rhs)
    {
        delete itsNum;
        itsNum = new int;
        *itsNum = *(rhs.itsNum);
    }
    
    return *this;
}

Also, another question is if pointers of *this and rhs point to the same address (even though two objects are distinct), doesn't deleting itsNum create a problem?

Of use?
http://parashift.com/c++-faq-lite/as...operators.html