1	Resource Management in C++ An alternative to garbage collection Bartosz Milewski www.relisoft.com
2	RAI in ARM Resource Acquisition Is Initialization Ellis, Stroustrup “The Annotated C++ Reference Manual” Addison-Wesley 1990
3	Bad Code!
4	Code Quality 1. Readability. Easy to read and understand. Not obscured by lots of inessential bookkeeping. Easy to factorize and assimilate one concept at a time. 2. Verifiability. Reasonably easy to verify correctness. Locality—you shouldn't have to refer to many separate pieces of code to prove correctness. Easy to debug. 3. Maintainability. Easy to modify—it shouldn't be fragile. A small modification shouldn't make it fail in unexpected ways. The creator or the maintainer should have as few opportunities as possible for making a mistake.
5	Bad Code In C
6	Transplanting Bad Code to C++
7	Good C++: Critical Section
8	Good C++: Lock
9	Good Clean C++: Usage
10	Resources Resource: “something” that must be released after it’s acquired Memory Heap objects created using new Handles: critical section, file, device context States: lock, reference count, progress UI (hourglass cursor) Objects: recursively
11	Resource Owners Owner: “something” that is responsible for the release of the resource Scopes (global, local)—automatic destruction of stack objects and global/static objects Objects: recursively—automatic destruction of sub-objects Built-in ownership mechanisms Destruction at the end of the scope Stack unwinding during exceptions Recursive destruction of sub-objects
12	First Rule of Acquisition Idea: make use of built-in mechs Acquire resources in constructors (one resource per object) Release resources in destructors
13	Heap Objects Note that heap objects are resources, so they must always have an owner object class Foo { public: Foo () : _bar (new Bar) {} ~Foo () { delete _bar; } private: Bar * _bar; };
14	Proof of correctness Gurantee that all resources have owners -> no resource leaks Raw resources owned by objects Objects owned by other objects and scopes Each scope is eventually exited, q.e.d. Note: Heap objects (allocated using new) are resources—must have owners Locality of proof—constructors/destructors
15	Ownership transfer mechanisms Built-in object transfer Returning objects by value Built-in object cloning Copy construction Assignment Passing arguments by value (a combination of the two) Copy constructors and operator= provide hooks to control transfers
16	Ownership Transfer in RM Idea: make use of built-in mechs Pass resources inside transfer objects with value-like semantics Example std::auto_ptr (move semantics) Ownership policies Disallow transfer: private copy-co/assignment Exclusive: auto_ptr-like Reference counted
17	Heap Resource Management Equivalent to Garbage Collection 99% can be solved by auto_ptr Allocation of heap resources auto_ptr<Foo> foo (new Foo); foo.reset (new Foo); : _foo (new Foo) // constructor Ownership transfer auto_ptr<Foo> MakeFoo (); void EatFoo (auto_ptr<Foo> foo);
18	Container for auto_ptr Standard containers don’t work with auto_ptr Standard containers work with ref-counted smart pointers (Alexandrescu) Must use RC pointers all over (-) No support in the standard yet (-) Overhead (-) Works with all containers (+)
19	Auto_vector (Milewski) Works with std::auto_ptr (big +) Little overhead (+) Must be generalized to other containers (-) [less needed as it might seem] Not supported by the standard (-)
20	Auto_vector A vector of pointers with modified interface and semantics
21	Auto_vector: lvalue
22	auto_lvalue
23	RM Garbage Collection Each heap object has an auto_ptr or auto_vector owner at all times Each polymorphic class has a virtual destructor Makes a very dependable cheap garbage collection mechanism But there is more!
24	Other resources Traditional garbage collection is useless with other resources Critical section and Lock example Windows resources Handles Files, memory mapped files Threads Etc.
25	Window Handle
26	Windows AutoHandle
27	Move Semantics
28	Default Disposal Policy Dipose is to be specialized
29	Icon
30	GDI Handle
31	Pen
32	Pen Maker
33	Device Context Handle
34	HDC Owners: PaintCanvas
35	HDC Owners: UpdateCanvas
36	Conclusion RM revolutionized our own work at Reliable Software Easy to enforce and maintain Global searches for new and delete Global searches for virtual Our own Windows library using RM More at www.ReliSoft.com