Relaxing constraints on complete types?

[lhmouse]

Relaxing constraints on complete types?

[lhmouse]

Yes and no.

It is not possible to define intrusive_ptr<Foo> where Foo is incomplete. However, we do have a workaround for this:

GIven class Foo, we fabricate a complete type

class Foo_fwd : public intrusive_base<Foo_fwd>
  {
    virtual ~Foo_fwd();   // remember to define this elsewhere.
  };

intrusive_ptr<Foo_fwd> foo_ptr;  // valid, because `Foo_fwd` is complete.

But it its intrusive_ptr<Foo> that we would like. We have to make Foo and Foo_fwd convertible to each other:

class Foo : public Foo_fwd
  {
    virtual ~Foo();  // remember to define this elsewhere.
  };

int main()
  {
    foo_ptr.reset(new Foo);  // conversion from `Foo*` to `Foo_fwd*` is valid.
    auto other = static_pointer_cast<Foo>(foo_ptr);  // static_cast'ing from base to derived is also valid.
    asserrt(foo_ptr == other);
  }

We also have a templated solution here: https://github.com/lhmouse/asteria/blob/fc67e16552943b40acd834fb5101b0b2ab6a5c4f/src/fwd.hpp#L235
The helper function that performs base-to-derived conversion is called unerase_cast.