pocketpy/docs/quick-start/misc.md

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The scope lock of gc

Sometimes you need to use the following code to prevent the gc from collecting objects.

auto _lock = vm->heap.gc_scope_lock();

The scope lock is required if you create a PyObject and then try to run python-level bytecodes.

For example, you create a temporary object on the stack and then call vm->py_str.

void some_func(VM* vm){
    PyObject* obj = VAR(List(5));
    // unsafe
    PyObject obj_string = vm->py_str(obj);
}

Because users can have an overload of __str__, this call is unsafe. When the vm is running python-level bytecodes, gc may start and delete your temporary object.

The scope lock prevents this from happening.

void some_func(VM* vm){
    PyObject* obj = VAR(List(5));
    // safe
    auto _lock = vm->heap.gc_scope_lock();
    PyObject obj_string = vm->py_str(obj);
}