Handling objects passed as "out parameters"

[madsmtm]

See Passing to an out parameter by writeback.

Some methods take pointers to an instance, and, if the pointer is not NULL, write some value to the pointer. Examples include:

• -[NSObject validateValue:forKey:error:]
• -[NSAppleScript initWithContentsOfURL:error:]
• -[NSCalendar rangeOfUnit:startDate:interval:forDate:]
• -[NSNetService getInputStream:outputStream:]
• Any error: (NSError**) err method

Note that out parameters work by autoreleasing the value - there may be a design where we embrace this, allowing us to cut down on some unnecessary retains and releases:

// Id<T, O> -> Id<T, O>
{
    let mut foo: Option<Id<T, O>>;
    // let old_foo = foo;
    msg_send![abc validateValue: &mut foo];
    // [old_foo release]
    // [foo retain]

    drop(foo)
}

// &T -> &T (easily possible today, but not very clear that the reference is bound to the pool)
pool {
    let mut foo: Option<&T>;
    msg_send![abc validateValue: &mut foo];

    // `foo` still usable inside the pool
}

// Id<T, O> -> &T
pool {
    let mut foo: Option<Id<T, O>>;
    // let old_foo = foo;
    msg_send![abc validateValue: &mut foo];
    // [old_foo release]
    let new_foo: &T = foo;
}

// &T -> Id<T, O>
pool {
    let mut foo: Option<&T>;
    msg_send![abc validateValue: &mut foo];
    // [foo retain]
    let new_foo: Id<T> = foo;
}

[madsmtm]

One implementation is the current Id::writeback (name up for bikeshedding).

But I'm wondering whether we could just allow passing &mut Option<Id<T, O>>/Option<&mut Option<Id<T, O>> in msg_send!, and let it automatically handle the required retain/release?

[madsmtm]

Also: Can we let it be automatically handled in declare_class!?