What does new (this) mean?

Question

I found this code sample for study:

T & T::operator=(T const & x)
{
  if (this != &x)
  {
    this->~T(); // destroy in place
    new (this) T(x); // construct in place
  }
  return *this;
}

When I look at the documentation for new there is no version that takes a pointer. Thus:

1. What does new (this) mean?
2. What is it used for?
3. How can it be called like this if it is not listed in the documentation?

Answer 1

It is called "placement new", and the comments in your code snippet pretty much explain it:

It constructs an object of type T without allocating memory for it, in the address specified in the parentheses.

So what you're looking at is a copy assignment operator which first destroys the object being copied to (without freeing the memory), and then constructs a new one in the same memory address. (It is also a pretty bad idea to implement the operator in this manner, as pointed out in the comments)