[spacechild1]

In practice, move operations typically just leave an empty object behind. The destructor already has to deal with that. And of course you can't call certain methods on an empty object. So in practice you don't need special logic except for the move operations themselves.

[Dylan16807]

> The destructor already has to deal with that.

That's partly true, partly circular. Because moves work this way, it's harder to make a class that doesn't have empty states, so I don't design my class to avoid empty states, so the destructor has to handle them.

[motorest]

> That's partly true, partly circular.

I don't think there is anything "partly" about it being true. A moved-from object is expected to remain valid and preserve class invariants. If you wrote a class whose objects fails to remain valid after being moved,you wrote bugs into your code.

> Because moves work this way, it's harder to make a class that doesn't have empty states, so I don't design my class to avoid empty states, so the destructor has to handle them.

You are not required to implement an empty state. You are only required to write your classes so that after moving an object it remains valid. You are free to specify what this means to your classes, and can be anything from leaving the object as if it was default initialized or have literally a member variable such as bool moved. It's up to you. In C++'s perspective as long as your moved-from object can be safely destroyed them it's all good. Anything else is the behavior you chose to have, and bugs you introduced.

[mort96]

It's not like it's the only part of the language that mandates a default constructor though. There are plenty of situations where default-constructible types are desirable. Even simple things like having a non-default-constructible type in a map is awkward.

[masklinn]

> It's not like it's the only part of the language that mandates a default constructor though

It’s… not a part of the langage which mandates a default ctor in the first place.

[motorest]

> It’s… not a part of the langage which mandates a default ctor in the first place.

Why should it, tough? Think about it. The goal of move semantics is performance, mainly avoiding to copy/initialize expensive objects using a standard syntax. Why do you believe it would be a good idea to force constructors when they can very well be the reason why move should be used?

[masklinn]

Did you reply to the wrong comment?

[mort96]

It doesn't, but it does mandate that the object has some "empty state". If you have an empty state you might as well have a default constructor which initializes the object to that empty state.

[masklinn]

moved-from objects are not in an empty state but in an unspecified state, they are only required to be destructible, every other operation can be disallowed. That is not a useful state for default construction. Thus being movable does not imply defaulting is any sort of good idea.

The other way around makes more sense, but even then it is not systematic, if default construction is costly (allocation, syscall, …) then you don’t want to do that for a moved-from object which will just be destroyed, which is the fate of most.

[mort96]

> moved-from objects are not in an empty state but in an unspecified state, they are only required to be destructible, every other operation can be disallowed. That is not a useful state for default construction. Thus being movable does not imply defaulting is any sort of good idea.

This is only true for standard library objects. The C++ standard specifies that e.g std::unordered_map will be in a "valid but unspecified state" when moved from. You can define your own classes to behave however you want, including defining a moved-from object to be identical to a default-constructed object.

Regardless, you're missing the point. Even if the standard specified that every moved-from object can only be destructed and every other use was UB, you still need the moved-from object to be in some empty state. Your move constructor and move operator= needs to put the moved-from object into some state where it doesn't own any resources but can be safely destructed. There's typically little reason to not make this "doesn't own any resources" state available through a default constructor.

> The other way around makes more sense, but even then it is not systematic, if default construction is costly (allocation, syscall, …) then you don’t want to do that for a moved-from object which will just be destroyed, which is the fate of most.

If your class can be in some "empty" state that doesn't own any resources and doesn't require syscalls to construct, you want this to be both the default constructed state and the moved-from state. Default-constructible objects end up getting default-constructed then overwritten all the time in C++, such as by the common pattern 'my_unordered_map["foo"] = MyClass(...)' which will first default-construct a value and then call its move operator=.

[spacechild1]

Please give me an example for a class that needs to handle empty state in the destructor only because of move operations. These exist, but IME they are very rare. As soon as you have a default constructor, the destructor needs to handle the case of empty state.

[ninkendo]

It’s not just the destructor you have to worry about, it’s all of the state accessible to callers.

If you have any type that represents validated data, say a string wrapper which conveys (say) a valid customer address, how do you empty it out?

You could turn it into an empty string, but now that .street() method has to return an optional value, which defeats the purpose of your type representing validated data in the first place.

The moved-from value has to be valid after move (all of its invariants need to hold), which means you can’t express invariants unless they can survive a move.

It is much better for the language to simply zap the moved-from value out of existence so that you don’t have to deal with any of that.

[spacechild1]

First, one shouldn't use a moved-from object in the first place (except for, maybe, reassigning it).

Second, why can't the .street() method simply return an empty string in this case?

> The moved-from value has to be valid after move (all of its invariants need to hold)

The full quote from the C++ standard is: "Unless otherwise specified, such moved-from objects shall be placed in a valid but unspecified state" AFAIK, it only makes such requirements for standard library types, but not for user defined types. Please correct me if I'm wrong.

[ninkendo]

> First, one shouldn't use a moved-from object in the first place (except for, maybe, reassigning it).

It still requires you to come up with somethkng to do to the old value in the move constructor. What would you do in the ValidatedAddress case? Set a flag in the struct called “moved_from” and use that to throw an exception if it’s ever used? Wouldn’t it be nice if you just didn’t need to worry about it?

> Second, why can't the .street() method simply return an empty string in this case?

In this example I’m referring to a type that represents a “validated” address, so, one that has already passed checks to make sure the street isn’t empty, etc. (it’s the whole “parse, don’t validate” idea, although I’ve never understood why the word “parse” is used when I would’ve just called it “validate just once”.)

It is an extremely useful concept for your type system to represents invariants in your data like this. Having to make every type contain an “empty” case, just to make the language’s move semantics work, pokes an enormous hole through this idea.

> AFAIK, it only makes such requirements for standard library types, but not for user defined types

It makes the requirement because the compiler is not going to stop anyone from using the moved-from value, so you have to think of something to do in the move constructor. You can pinky-swear to never use the moved-from value in your own code (and linters can help here) but the possibility still exists, so it must be solved for.

[spacechild1]

> Having to make every type contain an “empty” case, just to make the language’s move semantics work, pokes an enormous hole through this idea.

Nobody says that the invariants must hold after the object has been moved-from! The only thing you need to do is make sure that the destructor can run and do the right thing.

> You can pinky-swear to never use the moved-from value in your own code (and linters can help here) but the possibility still exists, so it must be solved for.

Letting the program crash would be a valid solution (for your own types).

For me the issue with C++ move semantics is not so much that you have to add special logic to your classes, but the fact that moved-from objects can be accessed in the first place. In this respect I definitely agree that destructive moves are better.

[motorest]

> Wouldn’t it be nice if you just didn’t need to worry about it?

Do you worry about it? I mean, to begin with, do you purposely try to reuse objects that you explicitly moved? If you do, in the very least you can be lazy and reassign a newly constructed object right after you explicitly move its contents, but I don't see any reason that would justify such a thing.

Can you point out what you feel is the scenario that worries you the most?

[tialaramex]

This means C++ is riddled with types that have unrelated "I'm empty" state inside them rather than this being relegated to a separate wrapper type. It's Tony's Billion Dollar Mistake but smeared across an entire ecosystem.

The smart pointer std::unique_ptr<T> is an example of this, sometimes people will say it's basically a boxed T, so analogous to Rust's Box<T> but it isn't quite, it's actually equivalent to Option<Box<T>>. And if we don't want to allow None? Too bad, you can't express that in C++

But you're right that C++ people soldier on, there aren't many C++ types where this nonsense unavoidably gets in your face. std::variant's magic valueless_by_exception is such an example and it's not at all uncommon for C++ people to just pretend it can't happen rather than take it square on.

[spacechild1]

> This means C++ is riddled with types that have unrelated "I'm empty" state

Again, these cases are still rare. Most classes either don't require user-defined move operations, or they have some notion of emptiness or default state.

> And if we don't want to allow None? Too bad, you can't express that in C++

That's actually a good example! Nitpick: you can express it in C++, just not without additional logic and some overhead :)

[7jjjjjjj]

>you can express it in C++, just not without additional logic and some overhead :)

How?

[spacechild1]

E.g. with a boolean member or by setting a bit in the pointer value.

[steveklabnik]

(And that difference leads to an ABI difference that makes it not a zero overhead abstraction in the way that Box is…)

[spacechild1]

Great point! Chandler Carruth explained this in one of this cppcon talks: https://youtu.be/rHIkrotSwcc?t=1047

[sgsjchs]

A socket.

[spacechild1]

How so? Doesn't your socket class have a default constructor and a notion of open and closed?

[sgsjchs]

If the moves were destructive, I'd design it to have the default constructor call `::socket` and destructor call `::close`. And there wouldn't be any kind of "closed" state. Why would I want it?

[spacechild1]

Your socket class would have no default constructor? And you would never want to close the socket before the object's lifetime ends? Really?