The Beauty of Destruction (Pete Isensee, Microsoft)

C++ Destructor Definition

• One
• Special
• Deterministic –> called at well defined times
• Automatic –> object out of scope or delete
• Symmetric –> constructor fits
• Member –> part of a class
• Function
• With
  • A special name (~)
  • No parameters
  • No return type
• Designed to
  • Give last rites
  • Before object death

C# uses finalizer different (called by GC) non-deterministic, same in Java

When destructors are called

• Global or static object, called when terminates
• Arrays destructed in reverse way
• STL container , unspecified order
• delete operator
• out of scope
• temp objects
• exception thrown (stack unwinding)
• explicitly
• exit()
• abort (does not call destructor)

Order of destruction

• Rule of thumb: Reverse order of construction
• Specifically
  • Destructor body
  • Data members in reverse order of declaration
  • Direct non-virtual base classes in reverse order
  • Virtual base classes in reverse order

Implicit Destructors

• not specified by programmer
• inline by default
• public
• recommended for struct-like POD-only objects
• for everything else, avoid implicit destructors
  • better debugging
  • improved perf analysis

Trivial Destructors

• Implicit
• Not virtual
• All direct base classes have trivial dtors
• All non-static members have trivial dtors
• Destructors that never do anything

Virtual Destructors

• Guarantee that derived classes get cleaned up
• Rule of thumb: if class has virtual functions, dtor should be virtual
  • if delete on Base* could ever point to a Derived*
• Perf: Obj with any virtual funcs includes a vtable ptr
• Idiom exceptions: mixin classes
• Pure signals abstract class (virtual ~T() = 0{})

Partial Construction & Destruction

• Dtors are only called for fully constructed objects
• if a ctor throws, obj was not fully constructed
  • obj dtor will not be called
  • but fully constructed subobjects will be destroyed
• Always use RAII with ctors
  • Resource Acquisition Is Initialization

Virtual Functions in Destructors

• Virtual functions are not virtual inside dtors

C++ Exception Handling

• Destructors : Exceptions :: Spock : Kirk
• Wrap any function that acquires a resource in a class where dtor releases the resource
• Never allow an exception to exit a dtor
  • Best: don’t throw in dtor
  • OK: wrap throwing code in a try/catch
• Good advice even if you don’t use C++EH

Multithreading

• You are responsible for protecting objects and their contents
• Sharing an object across threads
  • Use shared_ptr
  • or some other reference counting
  • or otherwise ensure only one thread can destroy
• Protect shared memory (global counters, ref counts) in dtor

delete and Destructors

• delete p is a two-step process

Explicit Destructors

• Destructors can be called directly
• Avoid 99.9% of the time
• Very powerful for custom memory scenarios
• Examples
  • w / placement new
  • STL allocators

std::allocator

• Allocators enable custom STL container memory
• Two key destructive functions

shared_ptr

• Templated non-intrusive deterministically referenced-counted smart pointer

shared_ptr deleters

• Deleter : a functor called on the stored raw pointer when ref count hits zero

Performance

• Destructors are called a LOT
• they are invisible in code
• streamline common dtors
• the best dtor is empty
• inlining
• profile