Is it possible to change a C++ object's class after instantiation?

Question

I have a bunch of classes which all inherit the same attributes from a common base class. The base class implements some virtual functions that work in general cases, whilst eac

Answer 1

I'm open to destroying the old object and creating a new one, as long as I can create the new object at the same memory address, so existing pointers aren't broken.

The C++ Standard explicitly addresses this idea in section 3.8 (Object Lifetime):

If, after the lifetime of an object has ended and before the storage which the object occupied is reused or released, a new object is created at the storage location which the original object occupied, a pointer that pointed to the original object, a reference that referred to the original object, or the name of the original object will automatically refer to the new object and, once the lifetime of the new object has started, can be used to manipulate the new object

Oh wow, this is exactly what you wanted. But I didn't show the whole rule. Here's the rest:

if:

• the storage for the new object exactly overlays the storage location which the original object occupied, and
• the new object is of the same type as the original object (ignoring the top-level cv-qualifiers), and
• the type of the original object is not const-qualified, and, if a class type, does not contain any non-static data member whose type is const-qualified or a reference type, and
• the original object was a most derived object (1.8) of type T and the new object is a most derived object of type T (that is, they are not base class subobjects).

So your idea has been thought of by the language committee and specifically made illegal, including the sneaky workaround that "I have a base class subobject of the right type, I'll just make a new object in its place" which the last bullet point stops in its tracks.

You can replace an object with an object of a different type as @RossRidge's answer shows. Or you can replace an object and keep using pointers that existed before the replacement. But you cannot do both together.

However, like the famous quote: "Any problem in computer science can be solved by adding a layer of indirection" and that is true here too.

Instead of your suggested method

Derived d;
Base* p = &d;
new (p) Base();  // makes p invalid!  Plus problems when d's destructor is automatically called

You can do:

unique_ptr p = make_unique();
p.reset(make_unique());

If you hide this pointer and slight-of-hand inside another class, you'll have the "design pattern" such as State or Strategy mentioned in other answers. But they all rely on one extra level of indirection.