Title: Seductive Tips

A few weeks ago the following appeared on one of the C++ newsgroups. I want you to study it very carefully and then itemise all the reasons why you consider this apparently seductive idea to be extremely dangerous.

A Not So Obvious C++ Tip:

Define copy assignment operator out of copy constructor Typically copy assignment operator and copy constructor are closely related to one another. Both of them are expected to leave an object in a state of being a copy to another one. One can use this fact in order to simplify the task of defining the assignment operator out of the copy constructor, as shown below.

//  Define copy assignment out of copy constructor.
//  DEFINE_ASS_BY_CTOR(ClassName)
//  To appear (only) within class definition.
// Define copy assignment out of copy constructor.
#define DEFINE_ASS_BY_CTOR(ClassName)  \
ClassName & operator = (const ClassName & from)  \ 
{                    \
  assert (this && &(from));                                 \
  assert (typeid (*this) == typeid (ClassName));   \
   if (this == &(from))                                      \
      return *this;                                          \
   ClassName::~ClassName();                                  \
   new (this) ClassName (from);                              \
   return *this;                                               \
   }