Weak References

The C++ backend by default creates objects using std::shared_ptr, a reference counted shared pointer. http://en.cppreference.com/w/cpp/memory/shared_ptr

Cyclic data structures will not work using only reference counted shared pointers. To break the cycle a weak pointer must be used, the child holds a weak pointer to its parent. The child then promotes the weak pointer to a reference counted pointer when it needs to use it, first checking if the pointer to the parent is still valid.

Rusthon will automatically detect when you need to use a weak pointer in an object, the only part you need to manage is promotion of the weak pointer by assigning it to a variable and testing if it is None (nullptr). This works because std::weak_ptr returns nullptr if the reference has been deleted. When using custom weakref types this syntax to check if a weak pointer is still alive may not work, you can instead use weak.valid(ptr) to check a weak reference, this style works with normal c++11 types and custom types. For more info on custom types, see: https://github.com/rusthon/Rusthon/wiki/Custom-Type-Templates

class Child:
  def __init__(self, p:Parent):
    self.parent = p
  def foo(self):
    parent = self.parent
    #if parent is None:     ## this style only works standard c++11 smart pointers
    if weak.valid(parent):  ## this works with custom types and standard c++11 smart pointers.
      print('do clean up...')
    else:
      do something with parent

You can also be more explicit by using weak.unwrap

parent = weak.unwrap(self.parent)

In order for Rusthon to automatically detect the reference cycle, the child must be stored in a list in the parent.

class Parent:
  def __init__(self, children:[]Child):
    self.children = children