Creating a new object from dynamic type info

Question

In C++, is there any way to query the type of an object and then use that information to dynamically create a new object of the same type?

For example, say I have a

Answer 1

There's only some hacky ways to do this.

The first and IMHO the ugliest is:

Base * newObjectOfSameType( Base * b )
{
  if( dynamic_cast( b ) ) return new Foo;
  if( dynamic_cast( b ) ) return new Bar;
}

Note that this will only work if you have RTTI enabled and Base contains some virtual function.

The second an neater version is to add a pure virtual clone function to the base class

struct Base { virtual Base* clone() const=0; }
struct Foo : public Base { Foo* clone() const { return new Foo(*this); }
struct Bar : public Base { Bar* clone() const { return new Bar(*this); }

Base * newObjectOfSameType( Base * b )
{
  return b->clone();
}

This is much neater.

One cool/interesting thing about this is that Foo::clone returns a Foo*, while Bar::clone returns a Bar*. You might expect this to break things, but everything works due to a feature of C++ called covariant return types.

Unfortunately covariant return types don't work for smart pointers, so using sharted_ptrs your code would look like this.

struct Base { virtual shared_ptr clone() const=0; }
struct Foo : public Base { shared_ptr clone() const { return shared_ptr(new Foo(*this) ); }
struct Bar : public Base { shared_ptr clone() const { return shared_ptr(new Bar(*this)); }

shared_ptr newObjectOfSameType( shared_ptr b )
{
  return b->clone();
}