Reference & Value Semantics

Reference Semantics

• Some languages default to using pass by reference.
  • AKA when an object is initialized from another object, or passed as a function argument, a reference is used.
• With this, the original variable cannot be destroyed until all other objects which are using it’s memory have been destroyed.

Garbage Collectors

• To manage the lifetimes of all of these objects, the language provides a garbage collector.
  • The program keeps track of all objects within it.
  • It also keeps track of which objects are using another object’s memory.
  • The program periodically stops executing and goes through all the objects and decides which memory allocations it is safe to release.
    • This is added overhead, requires more processing and memory.
    • It can’t execute any instructions while the garbage collector is running.
  • This also means memory cannot be released immediately.
  • We cannot predict the order/timing of when objects are destroyed.

C++ Specific Semantics

• Uses pass by value by default.
• If we want reference semantics, we can have them.
  • Heap allocated objects can exist beyond the end of the scope (think malloc or smart pointers).