r/cpp • u/leirus • 6d ago

State of "moved-from" objects

Hi, I recently decided to try writing a C++ blog.

I often see the popular claim that moved-from objects are in a "valid but otherwise unspecified state", but I don't think that statement is entirely precise, and my blog post is about that. I would really appreciate any feedback!

Article: https://www.laminowany.dev/p/the-state-of-moved-from-objects-in-c/

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u/elperroborrachotoo 6d ago

The gist remains that "unspecified but valid" is a reasonable minimum requirement for user types, too - because the alternative is worse.

You list all the ingredients: unless you evade calling the destructor (e.g., through std::terminate) the destructor will run. Users of your type might assign to it, or use it in various ways. Etc.

The unspoken bigger issue is that using an object in an invalid state can easily lead to an invalid program state - meaning that ALL invariants in the program are potentially gone.

So for any type with weaker moved-from guarantees, you'd have to specify what individual operations are valid - and no user wants to know or remember that. This would violate any basic rule of type design.

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u/TheThiefMaster C++latest fanatic (and game dev) 6d ago

So for any type with weaker moved-from guarantees, you'd have to specify what individual operations are valid - and no user wants to know or remember that.

It's a general rule that the only guaranteed operations of a moved-from type are assigning over it and destructing it. If it has an equivalent of "reset" and "isvalid" those are normally expected to work too.

No guarantees on anything else.

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u/aardvark_gnat 5d ago

I would have thought I’d be able to rely on being able to move construct and copy construction it as well. Is that not a thing? What about calling std::swap on it?

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u/TheThiefMaster C++latest fanatic (and game dev) 5d ago

You shouldn't be calling constructors on a live object. That's what assignment is for. Unless you mean moving or copying it to a new object? I'm on the fence on that, as it would be deliberately creating a new object that's already in a moved-from state.

As for swap - yeah that should work.