Smart Pointers

Dumb Pointers Pointers Necessary – Dynamic memory – Sharing memory

Dumb Pointers Pointers Necessary – Dynamic memory – Sharing memory But they are tricky – Memory leaks – Bad pointers

References References : safer pointers – C++ : Non-nullable

References Java : – All objects are on heap – Stack has Numeric Variables References – References are nullable – Cannot delete memory

References Python Everything is a reference… even numbers x = 1 x 1

References Python Everything is a reference… even numbers x = 1 x = 2 x 1 2

References Python Everything is a reference… even numbers x = 1 x = 2 y = x x 1 2 y

References Python Everything is a reference… even numbers x = 1 x = 2 y = x x = 4 x 1 2 y 4

References Python Everything is a reference… even numbers x = 1 x = 2 y = x x = 4 x = x + 1 x 1 2 y 4 5

Garbage Collection Garbage collection: Automatically reclaim unused memory from heap Various strategies Reference counting Mark and Sweep

Heap of Fish bonn.de/IV/martini/Lehre/Veranstaltungen/SS00/InformatikII /JavaSimulation/HeapOfFish.html bonn.de/IV/martini/Lehre/Veranstaltungen/SS00/InformatikII /JavaSimulation/HeapOfFish.html

Compaction Yellow fish are garbage:

Still Problem Still can't make a new Red Fish

Much Better Compacting "live" objects reduces fragmentation:

Smart Pointers C++11 brought smart pointers – Pointers do reference counting – Memory released when last reference is released

Shared Ptr shared_ptr : counts number sharing the object – When shared_ptr leaves scope, count-- – Memory deleted when count == 0

Weak Ptr weak_ptr : keeps track of object without "counting" – Can not be used directly – Use to ask for a real shared ptr when needed – Used to avoid cycles in shared_ptr

Unique Ptr unique_ptr : unshared pointer – Can't copy can only move – Deletes memory when pointer leaves scope

Courses With Strong/Weak Course Manager responsible for loading from DB

Courses With Strong/Weak Student asks for pointers to courses Course manager creates objects, stores weak_ptr

Courses With Strong/Weak Student asks for pointers to courses Student given shared_ptrs

Courses With Strong/Weak Student asks for pointers to courses Student given shared_ptrs

Courses With Strong/Weak Student asks for pointers to courses New student asks for a new class

Courses With Strong/Weak Student asks for pointers to courses New student asks for an existing class CS162 now has refcount of 2

Courses With Strong/Weak Student asks for pointers to courses First student destroyed… shared_ptrs with refcount == 1 destroyed

Objective C Can pick: – Manual release – Garbage collection (on some platforms) – ARC : Automatic Reference Counting Like shared_ptrs and weak_ptrs