[sgsjchs]

> what difference would it make

The same difference as making pointers always non-nullable and reintroducing nullability via an optional wrapper only when semantically appropriate.

> what could you possibly do with an arbitrary user-defined class

Just add some customization points to std::optional so that users can define which value of the class to treat as noneopt internally.

[spacechild1]

> The same difference as making pointers always non-nullable and reintroducing nullability via an optional wrapper only when semantically appropriate.

Again, I don't see what this has to do with destructive moves. If you want a socket class that always refer to an open socket, you can already do that. Same for non-nullable pointer wrappers. Conversely, destructive moves don't prevent you from implementing a socket class with a close() method. These concepts are really orthogonal.

> Just add some customization points to std::optional so that users can define which value of the class to treat as noneopt internally.

How is this supposed to work? The very point of your socket class is that it always contains a valid socket handle. Once you introduce a sentinel value, you are back to square one. If the optional class is able to construct a socket with the sentinel value, so is the user.

[sgsjchs]

> Again, I don't see what this has to do with destructive moves. If you want a socket class that always refer to an open socket, you can already do that.

Technically you can, but it's unreasonable to create an os-level socket just to put into the moved-out object where it will be immediately destroyed again. This is not an issue when the moves are destructive.

> How is this supposed to work? The very point of your socket class is that it always contains a valid socket handle. Once you introduce a sentinel value, you are back to square one. If the optional class is able to construct a socket with the sentinel value, so is the user.

That's not true. The sentinel value need not be exposed in the public interface of the class, it can only be accessible via the customization point of the optional.

[spacechild1]

> Technically you can, but it's unreasonable to create an os-level socket just to put into the moved-out object where it will be immediately destroyed again. This is not an issue when the moves are destructive.

No, the class can use a sentinel value internally only to mark moved-from objects. That's exactly where we actually started the conversation. That's why I said that destructive moves would only somewhat simplify the move operations, but not make a qualitative difference (in this area).

> The sentinel value need not be exposed in the public interface of the class, it can only be accessible via the customization point of the optional.

Since the optional would need to construct an instance with the sentinel value, I thought that the "sentinel" constructor must be public. However, you might be right that one could write a template specialization that contains the template argument as a friend class. In this case you could use a private constructor. Note that the destructor still has to handle the sentinel value... But I guess this is just something you have to accept.

[sgsjchs]

> No, the class can use a sentinel value internally only to mark moved-from objects. That's exactly where we actually started the conversation.

The issue is that the "moved-from" state is exposed to the user when the moves are not destructive. The author of the class has to consider behavior for every method in sentinel state, even when it's just to assert that the state isn't sentinel or "lol it's UB". And the user has to be careful not to accidentally misuse an object in sentinel state. Just like how every time you touch a nullable pointer you have to consider if it can be null and what to do in that case. As long as the sentinel state is exposed at all (via non-destructive move), there is little gain in not providing full support for it. However, with destructive moves the sentinel value either doesn't exist at all or only exists completely internally as an optimization, and all this mental overhead disappears.

[spacechild1]

I see your point. Just a few things:

1. This is only relevant when using such class as a local variable. Member variables are typically not moved-from.

2. In my understanding the user has the freedom to specify what constitutes a "valid but unspecified state" and it would be perfectly ok to mandate that anything you can do with a moved-from object is to either destroy or reassign it.

3. The problems with the state of moved-from objects from the perspective of a library author could have been prevented simply by imposing stricter requirements in the standard (e.g. every usage except destruction, and possible reassignment, shall be UB).

4. With all the issues you've pointed out, it is still be perfectly possible and reasonable to design a socket class your way (= no closed socket state) in C++, yet somehow most people seem to prefer open() and close() methods instead of modelling the state with an optional. Even in the presence of destructive moves, I don't think that one way is necessarily better than the other and it is mostly a matter of culture and personal preference.

All the being said, I definitely agree that destructive moves are good thing, in particular if the compiler prevents you accidentally accessing moved-from objects (which is a mistake that is very easy to make in C++).

[sgsjchs]

Indeed, the "valid but unspecified state" refers only to some types defined in the he standard library. It essentially means that you can only call methods which have no preconditions and don't depend on what that state is, e.g. assignment or destruction, or something like string::clear or vstring::assign if you want defined outcomes. In general each type is free to guarantee whatever the author wants about the moved from state, e.g. moved-from std::unique_ptr is always null.