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0 pointswwright6y ago0 comments

Rust has the exact same semantics there. Drop is useful when you need to explicitly notate that a value should end its life early.

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9 comments · 2 top-level

klipt6y ago· 5 in thread

void drop(unique_ptr<T>&& x) {}

Would do the same in C++ for values held by unique_ptr: take ownership of the pointer and then free it.

SamReidHughes6y ago

No, it would not free it. It takes a reference to the pointer and does nothing.

dkersten6y ago

unique_ptr disposes of the object its holding when it goes out of scope unless passed to another unique_ptr or ownership is explicitly released. Presumably klipt would std::move the unique_ptr, hence the universal reference (which seems unnecessary, just pass it by value).

Am I missing something? Does it not work with std::move?

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MauranKilom6y ago

It would have to be

void drop(std::unique_ptr<T> x) {}

Or, you know, just call someX.reset(); or do someX = nullptr;. The drop call would be way noisier: drop(std::move(someX));

edflsafoiewq6y ago

The equivalent to drop in C++ would be

    template <class T>
    void drop(T&) = delete; // force callers to move

    template <class T>
    void drop(T&& x) {
        T drop_me(std::move(x));
    }

edit: fixed.

Rusky6y ago

This is not correct; moves must leave the value in a valid state, because its destructor will still run. The correct version is actually the same as Rust's:

    template <class T>
    void drop(T) {}

(Moving into a local in `drop(T&&)` also works.)

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Filligree6y ago· 2 in thread

It used to, they got subtler. See https://stackoverflow.com/questions/50251487/what-are-non-le...

Rusky6y ago

NLL does not change those semantics; drop still runs at the end of the scope. It counts as a "use" for the purposes of NLL and thus keeps values with destructors alive.

steveklabnik6y ago

For some history, it was talked about changing this, but we couldn’t, due to back compatibility and it wasn’t clear it was actually a good idea. This was called “wary drop”.

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