Question

In a function, how to you assign this a new value?

Answer 1

You can’t.

9.3.2 The this pointer [class.this]

1 In the body of a non-static (9.3) member function, the keyword this is a prvalue expression whose value
is the address of the object for which the function is called. […] (emphasis & link mine)

You can modify the object this points to, which is *this. For example:

struct X
{
   int x;
   void foo()
   {
     this->x =3;
   }
};

The method modifies the object itself, but something like this = new X is illegal.