How to reassign `this` pointer inside object member function?

Question

I have an interesting question about C++ pointers.

You probably will think that I have to change my design, and avoid doing what I am doing, and you are probably right. But let's assume that I have a good reason to do it my way.

So this is the situation. I have a C++ class TestClass, and I have a pointer A of this type:

TestClass* A = new TestClass();

Among other things TestClass has this function:

void TestClass::Foo(){
    TestClass* B = new TestClass();
    ...
}

This function creates object B of the same type and populates it with some data.

At the end of this function, I want pointer A to point at object B. Anywhere outside this function it would look like A=B; inside this function it could look like this = B
But as you know you cannot reassign "this" pointer.

Possible solutions:

1. Copy the memory:

   memcpy(this, B, sizeof(TestClass));
   

   This method works correctly. The function copies each bit of object B into object A.
   Problem: if TestClass is a big object(and it is), it creates significant overhead in performance for multiple Foo calls.

2. Return a B pointer from the function and do something like this

   Temp = A;
   A=A->Foo();
   freeMemory(Temp);
   

   But this code looks stupid, and it makes function Foo very hard to use.

So the question is, how I can do this = B from inside a member function, without copying whole objects?

Answer 1

Use an extra level of indirection. Your TestClass can have a pointer that points to a class that contains all of its data.

class TestClass
{
private:
  TestClassData* m_data;

};

void TestClass::Foo()
{
  TestClassData* B = new TestClassData();
  ... 
  delete m_data;
  m_data = B;
} 

Just make sure your operator== returns true if the contents of m_data are equal.