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memory – PHP Generators – Garbage Collection

Posted by: admin July 12, 2020 Leave a comment


Simple question.

When or how, by PHP or yourself do generators destroy their stacks?

Take the following example:

function doWork(): Generator
    // create some objects.
    $o1 = new stdClass();
    $o2 = new stdClass();

    // pause here and wait for data.
    $value = yield 1;

    // By referencing the above objects, they shouldn't destruct.
    $o1->property = $value;
    $o2->property = $value;

    yield $o1;
    yield $o2;

    // End of stack.

// Create the generator.
$generator = doWork();

$value = $generator->current(); // $value will equal 1.

if ($x) {
    $generator->send('Hello, World!'); // Continue execution of the generator.
    $o1 = $generator->current();
    $o2 = $generator->current();
    $generator->next(); // Complete the generator

} else {
    // Do nothing with the generator.

// Carry on with script ...

In this example a generator is started and two objects are created. At this point it is yielded, and further data is requested.

An IF statement is reached.

Case 1

If $x is true, the value "Hello, World!" will be sent to the generator and the objects will be populated with their new properties.

The next time the yielded data is read, the objects will be returned.

Case 2

If $x is false, the generator will no longer be used.


In case 1, I would expect the stack to close like any other function, but what happens to the generator in case 2? Does it and all of the remaining object references stay in memory until the script ends?

Or does the loss of reference to $generator cause it, and all references inside to be cleared out?

How to&Answers:

There are two conditions under which a generator destroys its execution context (which also includes the variable table):

  1. If the generator finishes execution. This may either happen by executing a return (including the implicit return at the end of the function) or through an uncaught exception during execution of the generator.
  2. If all references to the generator object are relinquished.

So, no, the generator will not live until the script ends. It will be destroyed once the $generator variable goes out of scope, at which point the generator will relinquish its references to the variable values and other execution state.

You can easily observe destruction order by creating a class that echos in the destructor and then instantiating this class into a local variable.