On 29/06/2022 23:31, Dan Ackroyd wrote:
Imagine some code that looks like this:
// Acquire some resource e.g. an exclusive lock.
$some_resource = acquire_some_resource();
$fn = fn () {
// Free that resource
$some_resource = null;
}
// do some stuff that assumes the exclusive
// lock is still active.
// call the callback that we 'know' frees the resource
$fn();
That's a not unreasonable piece of code to write
For that to work, it would require the variable to be captured by
reference, not value. Writing to a variable captured by value, like
writing to a parameter passed by value, is just writing to a local variable.
In fact, the "optimisation" is in my opinion a critical part of the
semantics, to avoid the opposite problem:
// Acquire some resource e.g. an exclusive lock.
$some_resource = acquire_some_resource();
$fn = fn () {
// Use a variable that happens to have the same name
// A naive implementation would see $some_resource mentioned, and
capture it
// Over-writing the local variable here makes no difference; the
closure still holds the value for next time
$some_resource = 'hello';
}
// Free what we believe is the last pointer, to trigger the destructor
unset($some_resource);
// If $some_resource gets captured, it can only be released by
destroying the closure
unset($fn);
Regards,
--
Rowan Tommins
[IMSoP]
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