| Member Functions | |
| allocate() deallocate() dirty() lock() pageSize() unlock() |
#include <rw/vpage.h> (Abstract base class)
This is an abstract base class representing an abstract page heap of fixed sized pages. The following describes the model by which specializing classes of this class are expected to work.
You allocate a page off the abstract heap by calling member function allocate() which will return a memory "handle," an object of type RWHandle. This handle logically represents the page.
In order to use the page it must first be "locked" by calling member function lock() with the handle as an argument. It is the job of the specializing class of RWVirtualPageHeap to make whatever arrangements are necessary to swap in the page associated with the handle and bring it into physical memory. The actual swapping medium could be disk, expanded or extended memory, or a machine someplace on a network. Upon return, lock() returns a pointer to the page, now residing in memory.
Once a page is in memory, you are free to do anything you want with it although if you change the contents, you must call member function dirty() before unlocking the page.
Locked pages use up memory. In fact, some specializing classes may have only a fixed number of buffers in which to do their swapping. If you are not using the page, you should call unlock(). After calling unlock() the original address returned by lock() is no longer valid -- to use the page again, it must be locked again with lock().
When you are completely done with the page then call deallocate() to return it to the abstract heap.
In practice, managing this locking and unlocking and the inevitable type casts can be difficult. It is usually easier to design a class that can work with an abstract heap to bring things in and out of memory automatically. Indeed, this is what has been done with class RWTValVirtualArray<T>, which represents a virtual array of elements of type T. Elements are automatically swapped in as necessary as they are addressed.
None
This example illustrates adding N nodes to a linked list. In this linked list, a "pointer" to the next node is actually a handle.
#include <rw/vpage.h>
struct Node {
int key;
RWHandle next;
};
RWHandle head = 0;
void addNodes(RWVirtualPageHeap& heap, unsigned N) {
for (unsigned i=0; i<N; i++){
RWHandle h = heap.allocate();
Node* newNode = (Node*)heap.lock(h);
newNode->key = i;
newNode->next = head;
head = h;
heap.dirty(h);
heap.unlock(h);
}
}
RWVirtualPageHeap(unsigned pgsize);
Sets the size of a page.
virtual ~RWVirtualPageHeap();
The destructor has been made virtual to give specializing classes a chance to deallocate any resources that they may have allocated.
unsigned pageSize() const;
Returns the page size for this abstract page heap.
virtual RWHandle allocate() = 0
Allocates a page off the abstract heap and returns a handle for it. If the specializing class is unable to honor the request, then it should return a zero handle.
virtual void deallocate(RWHandle h) = 0;
Deallocate the page associated with handle h. It is not an error to deallocate a zero handle.
virtual void dirty(RWHandle h) = 0;
Declare the page associated with handle h to be "dirty." That is, it has changed since it was last locked. The page must be locked before calling this function.
virtual void* lock(RWHandle h) = 0;
Lock the page, swapping it into physical memory, and return an address for it. A nil pointer will be returned if the specializing class is unable to honor the lock. The returned pointer should be regarded as pointing to a buffer of the page size.
virtual void unlock(RWHandle h) = 0;
Unlock a page. A page must be locked before calling this function. After calling this function the address returned by lock() is no longer valid.