A Simple Garbage Collector for C++ - An Overview of the Garbage Collector Classes
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The garbage collector uses four classes: GCPtr, GCInfo, Iter, and OutOfRangeExc. Before examining the code in detail, it will be helpful to understand the role each class plays.
GCPtr At the core of the garbage collector is the class GCPtr, which implements a garbage-collection pointer. GCPtr maintains a list that associates a reference count with each piece of memory allocated for use by a GCPtr. In general, here is how it works. Each time a GCPtr is pointed at a piece of memory, the reference count for that memory is incremented. If the GCPtr was previously pointing at a different piece of memory prior to the assignment, the reference count for that memory is decremented. Thus, adding a pointer to a piece of memory increases the memory’s reference count. Removing a pointer decreases the memory’s reference count. Each time a GCPtr goes out of scope, the reference count associated with the memory to which it currently points is decremented. When a reference count drops to zero, that piece of memory can be released.
GCPtr is a template class that overloads the * and –> pointer operators and the array indexing operator [ ]. Thus, GCPtr creates a new pointer type and integrates it into the C++ programming environment. This allows a GCPtr to be used in much the same way that you use a normal C++ pointer. However, for reasons that will be made clear later in this chapter, GCPtr does not overload the ++, – –, or the other arithmetic operations defined for pointers. Thus, except through assignment, you cannot change the address to which a GCPtr object points. This may seems like a significant restriction, but it isn’t because the Iter class provides these operations.
For the sake of illustration, the garbage collector runs whenever a GCPtr object goes out of scope. At that time, the garbage collection list is scanned and all memory with a reference count of zero is released, even if it was not originally associated with the GCPtr that went out of scope. Your program can also explicitly request garbage collection if you need to recycle memory earlier.
GCInfo As explained, GCPtr maintains a list that links reference counts with allocated memory. Each entry in this list is encapsulated in an object of type GCInfo. GCInfo stores the reference count in its refcount field and a pointer to the memory in its memPtr field. Thus, a GCInfo object binds a reference count to a piece of allocated memory.
GCInfo defines two other fields: isArray and arraySize.If memPtr points to an allocated array, then its isArray member will be true and the length of the array will be stored in its arraySize field.
The Iter Class As explained, a GCPtr object allows you to access the memory to which it points by using the normal pointer operators * and –>, but it does not support pointer arithmetic. To handle situations in which you need to perform pointer arithmetic, you will use an object of type Iter. Iter is a template class similar in function to an STL iterator, and it defines all pointer operations, including pointer arithmetic. The main use of Iter is to enable you to cycle through the elements of a dynamically allocated array. It also provides bounds checking. You obtain an Iter from GCPtr by calling either begin( ) or end( ), which work much like their equivalents in the STL.
It is important to understand that although Iter and the STL iterator type are similar, they are not the same, and you cannot use one in place of the other.
OutOfRangeExc If an Iter encounters an attempt to access memory outside the range of the allocated memory, an OutOfRangeExc exception is thrown. For the purposes of this chapter, OutOfRangeExc contains no members. It is simply a type that can be thrown. However, you are free to add functionality to this class as your own applications dictate.
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 This article was excerpted from chapter two of The Art of C++, written by Herbert Schildt (McGraw-Hill/Osborne, 2004; ISBN: 0072255129). Check it out at your favorite bookstore today. Buy this book now.
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