| Member Functions | |
| heap() length() operator=() operator[]() reshape() set() |
slice() val() |
#include <rw/tvrtarry.h> RWVirtualPageHeap* heap; RWTValVirtualArray<T> array(1000L, heap);
This class represents a virtual array of elements of type T of almost any length. Individual elements are brought into physical memory as needed basis. If an element is updated it is automatically marked as "dirty" and will be rewritten to the swapping medium.
The swap space is provided by an abstract page heap which is specified by the constructor. Any number of virtual arrays can use the same abstract page heap. You must take care that the destructor of the abstract page heap is not called before all virtual arrays built from it have been destroyed.
The class supports reference counting using a copy-on-write technique, so (for example) returning a virtual array by value from a function is as efficient as it can be. Be aware, however, that if the copy-on-write machinery finds that a copy must ultimately be made, then for large arrays this could take quite a bit of time.
For efficiency, more than one element can (and should) be put on a page. The actual number of elements is equal to the page size divided by the element size, rounded downwards. Example: for a page size of 512 bytes, and an element size of 8, then 64 elements would be put on a page.
The indexing operator (operator[](long)) actually returns an object of type RWTVirtualElement<T>. Consider this example:
double d = vec[j]; vec[i] = 22.0;
Assume that vec is of type RWTValVirtualArray<double>. The expression vec[j] will return an object of type RWTVirtualElement<double>, which will contain a reference to the element being addressed. In the first line, this expression is being used to initialize a double. The class RWTVirtualElement<T> contains a type conversion operator to convert itself to a T, in this case a double. The compiler uses this to initialize d in the first line. In the second line, the expression vec[i] is being used as an lvalue. In this case, the compiler uses the assignment operator for RWTVirtualElement<T>. This assignment operator recognizes that the expression is being used as an lvalue and automatically marks the appropriate page as "dirty," thus guaranteeing that it will be written back out to the swapping medium.
Slices, as well as individual elements, can also be addressed. These should be used wherever possible as they are much more efficient because they allow a page to be locked and used multiple times before unlocking.
The class T must have:
well-defined copy semantics (T::T(const T&) or equiv.);
well-defined assignment semantics (T::operator=(const T&) or equiv.).
In addition, you must never take the address of an element.
None
In this example, a virtual vector of objects of type ErsatzInt is exercised. A disk-based page heap is used for swapping space.
#include <rw/tvrtarry.h>
#include <rw/rstream.h>
#include <rw/diskpage.h>
#include <stdlib.h>
#include <stdio.h>
struct ErsatzInt {
char buf[8];
ErsatzInt(int i) { sprintf(buf, "%d", i); }
friend ostream& operator<<(ostream& str, ErsatzInt& i)
{ str << atoi(i.buf); return str; }
};
main() {
RWDiskPageHeap heap;
RWTValVirtualArray<ErsatzInt> vec1(10000L, &heap);
for (long i=0; i<10000L; i++)
vec1[i] = i; // Some compilers may need a cast here
cout << vec1[100] << endl; // Prints "100"
cout << vec1[300] << endl; // Prints "300"
RWTValVirtualArray<ErsatzInt> vec2 = vec1.slice(5000L, 500L);
cout << vec2.length() << endl; // Prints "500"
cout << vec2[0] << endl; // Prints "5000";
return 0;
}
Program output:
100 300 500 5000
RWTValVirtualArray<T>(long size, RWVirtualPageHeap* heap);
Construct a vector of length size. The pages for the vector will be allocated from the page heap given by heap which can be of any type.
RWTValVirtualArray<T>(const RWTValVirtualArray<T>& v);
Constructs a vector as a copy of v. The resultant vector will use the same heap and have the same length as v. The actual copy will not be made until a write, minimizing the amount of heap allocations and copying that must be done.
RWTValVirtualArray<T>(const RWTVirtualSlice<T>& sl);
Constructs a vector from a slice of another vector. The resultant vector will use the same heap as the vector whose slice is being taken. Its length will be given by the length of the slice. The copy will be made immediately.
~RWTValVirtualArray<T>();
Releases all pages allocated by the vector.
RWTValVirtualArray& operator=(const RWTValVirtualArray<T>& v);
Sets self to a copy of v. The resultant vector will use the same heap and have the same length as v. The actual copy will not be made until a write, minimizing the amount of heap allocations and copying that must be done.
void operator=(const RWTVirtualSlice<T>& sl);
Sets self equal to a slice of another vector. The resultant vector will use the same heap as the vector whose slice is being taken. Its length will be given by the length of the slice. The copy will be made immediately.
T operator=(const T& val);
Sets all elements in self equal to val. This operator is actually quite efficient because it can work with many elements on a single page at once. A copy of val is returned.
T operator[](long i) const;
Returns a copy of the value at index i. The index i must be between zero and the length of the vector less one or an exception of type TOOL_LONGINDEX will occur.
RWTVirtualElement<T> operator[](long);
Returns a reference to the value at index i. The results can be used as an lvalue. The index i must be between zero and the length of the vector less one or an exception of type TOOL_LONGINDEX will occur.
long length() const;
Returns the length of the vector.
T val(long i) const;
Returns a copy of the value at index i. The index i must be between zero and the length of the vector less one or an exception of type TOOL_LONGINDEX will occur.
void set(long i, const T& v);
Sets the value at the index i to v. The index i must be between zero and the length of the vector less one or an exception of type TOOL_LONGINDEX will occur.
RWTVirtualSlice<T> slice(long start, long length);
Returns a reference to a slice of self. The value start is the starting index of the slice, the value length its extent. The results can be used as an lvalue.
void reshape(long newLength);
Change the length of the vector to newLength. If this results in the vector being lengthened then the value of the new elements is undefined.
RWVirtualPageHeap* heap() const;
Returns a pointer to the heap from which the vector is getting its pages.