Click here to Skip to main content
Click here to Skip to main content

AGM::LibGC: a C++ garbage collection library

, 30 Jul 2004
Rate this:
Please Sign up or sign in to vote.
AGM::LibGC is a C++ garbage collection library.

Introduction

LibGC is a very small library (around 500 lines of code) that gives garbage collection capabilities to C++. It is developed, tested, and used under MSVC++ 6.0.

The usual solution for memory management in the C++ environment (aside from using garbage collectors, like the Hans - Boehm one) is reference counting. But reference counting has some fundamental problems:

  1. cyclic references
  2. slow speed of execution
  3. difficulty in programming

The problem no. 1) is due to having a counter that counts how many other objects point to a certain object; the counter of the object must reach 0 in order for the object to be released. Unfortunately, when one object A points to another object B and that object B points back to A, these two objects will never be released since there is a cycle between them.

The problem no. 2) is due to the way reference counting works. Within every assignment, the managed pointer class has to do the following steps:

  1. lock resources (if the solution is multithreaded)
  2. compare current pointer with new one; if there is no change, do nothing
  3. store the current pointer value in a temporary variable
  4. copy the value of the parameter pointer to the pointer member
  5. increase the reference count of the new object
  6. decrease the reference count of the previous object
  7. delete the previous object, if the reference count reaches 0
  8. unlock resources (if the solution is multithreaded)

All the above operations take place in every assignment; they are very expensive and can make a program very slow, especially if the program has thousands of pointers (for example, on the stack).

The problem no. 3) is due to the way reference counting works. Special care should be taken in handcrafting destructors in order to avoid multiple deletions of the same memory block, a task that is quite complicated. Many programmers tend to make wrapper classes around shared objects in order to avoid this problem, but then they have to manage two classes with the same interface: the wrapper class and the internal class.

The solution offered by LibGC has none of these disadvantages. Objects that are referenced in cycles are collected and deleted normally, the library is generally quite fast in doing garbage collection, and there is no need for wrapper classes.

LibGC uses the conservative mark-and-sweep stop-the-world algorithm to collect the garbage data. The programmer writes C++ as usual, with pointers and such, but objects need not be deleted. Object deletions work as usual.

The license of LibGC is the LGPL.

Using LibGC is very simple; just program normally as you always did:

  • allocate objects with operator new.
  • allocate objects that need finalization with macro GC_NEW.
  • inherit your classes from class Object in order to be automatically finalized without the usage of GC_NEW.
  • if you need to delete objects, delete them with operator delete.
  • declare objects on the stack.
  • declare objects globally (in the program's data section).
  • manually do garbage collection by calling the function doGC().

Deletions work normally: you can delete an object anytime, and the memory occupied by the object is no longer garbage-collected.

When the application exits, all objects are finalized, and memory is freed.

The class Object is in the namespace agm::gc.

Example:

//use garbage collection
#include "gc.hpp"
using namespace agm::gc;

//include some STL class for demonstrating the global use of operator 'new'
#include <list>
using namespace std;

//a class that is not automatically finalized
class Foo {
public:
};

//a class that is automatically finalized
class Bar : public Object {
public:
};

//main
int main()
{
    //allocate a Foo object with macro GC_NEW 
    //because it is not automatically finalized
    Foo *obj1 = GC_NEW(Foo)();

    //allocate an STL list that is not automatically finalized;
    //nodes will be freed by the collector
    list<INT> *obj2 = new list<INT>;

    //allocate a Bar object with operator 'new' 
    //that is automatically finalized
    //since it inherits from Object
    Bar *obj3 = new Bar;
}

The library will collect garbage automatically as soon as more than GC_THRESHOLD bytes have not been collected.

  • Note 1: The library has been developed and tested under MSVC++ 6.0. It will probably run under other compilers, too. If you want to test it and tell me, feel free to do so: you will be added in the list of contributors.
  • Note 2: Exceptions must be enabled.
  • Note 3: The solution does not work for multithreaded applications (yet).

For more information, you may visit my little site here.

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here

Share

About the Author

Achilleas Margaritis
Software Developer (Senior)
Greece Greece
No Biography provided

Comments and Discussions

 
GeneralLibGC error Pinmembercaoctaviano25-Jun-09 9:46 
GeneralProblem with libgc Pinmemberpulkownik1126-Mar-08 23:37 
QuestionA question in reading your code Pinmembernightelf19843-Jun-07 21:03 
QuestionIs it thread safe? PinmemberAliff9-Oct-04 19:07 
GeneralAn interesting possible problem PinmemberNathan Holt at CCEI3-Aug-04 7:07 
GeneralRe: An interesting possible problem Pinmemberaxilmar3-Aug-04 23:49 
GeneralRe: An interesting possible problem PinmemberEdwin G. Castro10-Aug-04 8:17 
GeneralMingW32 support. Pinmemberaxilmar3-Aug-04 6:19 
GeneralGarbage PinmemberTW2-Aug-04 16:33 
GeneralRe: Garbage Pinmemberaxilmar3-Aug-04 0:45 
GeneralRe: Garbage PinmemberNemanja Trifunovic3-Aug-04 5:58 
GeneralRe: Garbage Pinmemberaxilmar3-Aug-04 6:21 
GeneralRe: Garbage PinmemberNathan Holt at CCEI3-Aug-04 6:53 
GeneralRe: Garbage PinmemberNemanja Trifunovic3-Aug-04 6:56 
GeneralRe: Garbage Pinmemberaxilmar3-Aug-04 23:52 
GeneralRe: Garbage PinmemberNathan Holt at CCEI4-Aug-04 5:47 
GeneralRe: Garbage Pinmemberaxilmar4-Aug-04 6:44 
GeneralRe: Garbage PinmemberNathan Holt at CCEI4-Aug-04 7:17 
GeneralRe: Garbage Pinmemberaxilmar5-Aug-04 0:05 
GeneralRe: Garbage PinmemberNathan Holt at CCEI5-Aug-04 6:44 
GeneralRe: Garbage Pinmemberaxilmar5-Aug-04 10:04 
GeneralRe: Garbage PinmemberNathan Holt at CCEI6-Aug-04 6:59 
GeneralRe: Garbage Pinmemberaxilmar8-Aug-04 0:40 
GeneralRe: Garbage PinmemberNathan Holt at CCEI9-Aug-04 6:48 
axilmar wrote:
I did not need the functionality of signals and slots, I needed to call a method of the parent. Signals and slots is a callback mechanism.
 
I agree that some classes have so little potential for reuse that there is no point in a signal/slot method. Is that what you were trying to say there?
 
axilmar wrote:
For example, in a linked list, a node may be finalized first (i.e. its destructor being called), but will not be freed. Then the list deletes the node, calls its destructor (the destructor is called twice, but this can not be avoided in any C++ GC because there is no control over destructor calling), but the node is still not freed. So we don't have the double deletion problem.
 
Calling destructors multiple times is exactly what I was refering to. In many classes, this can be done safely, but there are reasonable situations in which this could be disastrous. If a destructor is virtual, for instance, the compiler changes the pointer to the virtual member table as the more derived parts of the object are destroyed, and I would consider it quite reasonable to set the pointer to NULL once an object is destroyed. Then, the second time the destructor was called, code would attempt to find the address of the virtual destructor by using a NULL pointer.
 
axilmar wrote:
By the way, do you know of any way that I can tell if a pointer is wild under Unix? i.e. if I used the pointer, it would cause a SIGSEV or SIGBUS; but I want to know that before I use it. On Windows I use structured exception handling and the functions IsBadReadPtr and IsBadWritePtr, but I can't find something similar for Unix. If you know something, I could port the collector to Unix.
 
I'm afraid it's been years since I programed in UNIX. I would not be suprised if the functions needed were different for each form of UNIX.
 
Nathan Holt
GeneralRe: Garbage PinmemberNemanja Trifunovic4-Aug-04 16:27 
GeneralRe: Garbage Pinmemberaxilmar5-Aug-04 0:06 
GeneralVersion 2.1 is out. Pinmemberaxilmar2-Aug-04 6:12 
GeneralDoes not work with MFC apps in Debug build PinmemberFD1-Aug-04 23:25 
GeneralRe: Does not work with MFC apps in Debug build Pinmemberaxilmar1-Aug-04 23:57 
GeneralQuick questions... PinmemberEd Preston1-Aug-04 21:28 
GeneralRe: Quick questions... PinmemberEd Preston1-Aug-04 21:50 
GeneralRe: Quick questions... Pinmemberaxilmar2-Aug-04 0:22 
GeneralMultithreaded GC PinmemberNathan Holt at CCEI3-Aug-04 5:32 
GeneralRe: Multithreaded GC Pinmemberaxilmar3-Aug-04 6:30 
GeneralRe: Quick questions... Pinmemberaxilmar2-Aug-04 0:04 
GeneralRe: Quick questions... PinmemberEd Preston2-Aug-04 2:35 
GeneralRe: Quick questions... Pinmemberaxilmar2-Aug-04 6:08 
GeneralRe: Quick questions... PinmemberJim Crafton3-Aug-04 3:07 
GeneralRe: Quick questions... PinsussAnonymous3-Aug-04 5:21 
GeneralRe: Quick questions... Pinmemberaxilmar3-Aug-04 6:22 
GeneralRe: Quick questions... Pinmemberaxilmar3-Aug-04 6:22 
Questioncould you please elaborate? Pinmemberpeterchen1-Aug-04 0:00 
AnswerRe: could you please elaborate? Pinmemberaxilmar1-Aug-04 0:34 

General General    News News    Suggestion Suggestion    Question Question    Bug Bug    Answer Answer    Joke Joke    Rant Rant    Admin Admin   

Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.

| Advertise | Privacy | Mobile
Web04 | 2.8.140814.1 | Last Updated 31 Jul 2004
Article Copyright 2004 by Achilleas Margaritis
Everything else Copyright © CodeProject, 1999-2014
Terms of Service
Layout: fixed | fluid