std::move'ing from a member variable

[Zentaro Kavanagh]

Is it still correct that the only thing we should expect from the moved-from type is that it is valid and safely destructible? I think we have something that checks for use-after-move which I think works for local variables, which implies we should not be moving from a member unless we ca n be sure it is never used again.

In the case where that can't be validated, or where we explicitly want to reuse for example a member vector full of move only objects, is swap the recommended way to do this?

eg.

std::vector ret; member_.swap(ret); return ret;

[dan...@chromium.org]

You can also reset the variable after move:

std::vector ret = std::move(member_);

member = std::vector<T>();

return ret;

Your understanding that we should not use-after-move is correct (with a few exceptions like smart pointers which allow it)

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[Roland McGrath]

There is also `std::exchange`: `return std::exchange(member_, {});` is often the most concise thing.

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[Lei Zhang]

For the vector case, item 8 on [1] says `member_` is guaranteed to be
empty. Should be fine to just return std::move(member_), right?

[1] https://en.cppreference.com/w/cpp/container/vector/vector

On Tue, Apr 5, 2022 at 9:22 AM <dan...@chromium.org> wrote:
>

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[dan...@chromium.org]

On Tue, Apr 5, 2022 at 7:53 PM Lei Zhang <the...@chromium.org> wrote:

For the vector case, item 8 on [1] says `member_` is guaranteed to be 

empty. Should be fine to just return std::move(member_), right?

Ah, looks right yeah, for vector. I didn't recall that vector defines that.

[Peter Kasting]

On Wed, Apr 6, 2022 at 6:29 AM <dan...@chromium.org> wrote:

On Tue, Apr 5, 2022 at 7:53 PM Lei Zhang <the...@chromium.org> wrote:

For the vector case, item 8 on [1] says `member_` is guaranteed to be 

empty. Should be fine to just return std::move(member_), right?

Ah, looks right yeah, for vector. I didn't recall that vector defines that.

Neither will anyone who reads the code.  It will look like a bug.

 I'd go the std::exchange() route myself.

PK

[Gabriel Charette]

I have a stalled proposal to do this automatically for member variables via base::ResetOnMove<T>.

This would allow operator=(Foo&&) = default; instead of forcing the explicit declaration of all move operations because a single member must be reset (default constructed) after move.

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