As another commenter says the moved-from object should have "Valid but unspecified state" (types provided by the standard library will do that, custom types merely should do that)

Since you don't know what valid state it has, calls with pre-requisites are nonsense (e.g. if you have a Bird and the method land requires that the Bird should be flying, you can't call land() on a moved from Bird, because you don't know if it's flying) but all calls without pre-requisites are fine e.g. asking how long a string you moved is would work - it's probably zero length now, but maybe not.

> Presumably it wouldn’t be hard for the compiler to yell at you.

In the general case this is Undecidable, so, the opposite of not hard.

> I would assume you could change this without affecting backwards compatibility.

C++ which relies on this exists today, the most likely path to actually landing destructive move in C++ would be to add a whole new set of construction and assignment operators for destructive move, forcing people to opt in and adding to the many sets C++ already has, and likely angering C++ developers a great deal in the process.

Howard Hinnant, whose design today's non-destructive move is, did argue that in principle it's possible to add destructive move to the language later if desired, but his description rather undersells the benefits of this design, presumably because he couldn't deliver it. Maybe he'd watched enough Mad Men (yup, Mad Men's early seasons pre-date C++ having move semantics) to know you shouldn't tell the customer what they can't have or they'll want it.

Common things to actually do with a C++ variable after moving from it are:

* Nothing, but in the knowledge it won't be cleaned up until the scope ends * Re-assign it, destroying the hollowed out object immediately * Re-use the hollowed out object, e.g. call a clear() method on it and then use as normal

The only requirement placed on the “moved out” variable is that you should be able to call its destructor. Which means that it has to be in a valid but unspecified state. So it's fine to access such a variable, so long as you don't read its exact state. You can still assign to it, for instance.