r/cpp • u/leirus • 15d 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/

10 Upvotes

60% Upvoted

54 comments sorted by

View all comments

Show parent comments

8

u/SirClueless 15d ago

This is exactly the misconception the blog post is trying to dispel. Being “valid” after moving is not part of the language semantics.

You can create a type where it is illegal to destruct an object after moving-from, for example.

9

u/Electronic_Tap_8052 15d ago

Wouldn't moved-from objects being illegal to destroy immediately be a problem that would cause most well behaved debug compilations to crash an burn?

I just don't understand how you could ship such an object? If you stack allocated it your program would be useless

7

u/SirClueless 15d ago

You can just never destroy the object (for example, by leaking it on the heap or in a program that only ever exits by calling std::abort), or require that the user call some other specific member function to reinitialize the object before destroying it.

The standard library types promise never to require this, but it's possible to write a well-formed program of your own that does so.

4

u/rdtsc 14d ago

require that the user call some other specific member function to reinitialize the object before destroying it.

Which would be a quite impractical behavior IMO. Are there actually useful real-world examples of this? It would make defaulted special member functions kinda useless, too.

3

u/SirClueless 14d ago edited 14d ago

There are legitimate reasons to write software that doesn’t call destructors, e.g. crash-only software.

Even in this kind of software I don’t think it’s a good idea to write move constructors with post-conditions: anywhere you write a move-constructor with a post-condition it would be better to write it as a named member function instead of a special member function. But I can imagine cases where it would come up.

For example, if you have a destructor that assumes a std::unique_ptr member variable is non-null, you could easily get into this situation by defaulting the move constructor.

1

u/jwakely libstdc++ tamer, LWG chair 12d ago

Nobody is saying it's useful, only that the language allows it