C++箴言：在operator= 中處理自賦值

C++箴言：在operator= 中處理自賦值
http://www.builder.com.cn/2007/1027/585438.shtml

w = w;
自賦值安全（self-assignment-safe）
異常安全（exception-safe）

=================================================================
Widget&
Widget::operator=(const Widget& rhs) // unsafe impl. of operator=
{
delete pb; // stop using current bitmap
pb = new Bitmap(*rhs.pb); // start using a copy of rhs's bitmap

return *this; // see Item 10
}

class Widget {
...　
void swap(Widget& rhs); // exchange *this's and rhs's data;
... // see Item 29 for details
};

================================================================
Widget& Widget::operator=(const Widget& rhs)
{
Widget temp(rhs); // make a copy of rhs's data

swap(temp); // swap *this's data with the copy's
return *this;
}