对对象的存储对每一种语言都很重要,例如C++中的Iterator, C#中的IEnumerator, Java中的Iterator等。
C++:所有的集合类都定义了一个叫iterator的数据成员。可以通过此对象实现对集合的traverse.
vector<int> ivec(10,42);
// loop using iterators to reset all the elements in ivec to 0
for(vector<int>:: iterator iter=ivec.begin(); iter != ivec.end(); ++iter)
*iter=0
C#:
public interface IEnumerator
{
bool MoveNext();
object Current { get; }
void Reset();
}
public interface IEnumerable
{
IEnumerator GetEnumerator();
}
最需要体会的一点是这里的设计原则符合优先使用composition而不是inhertiance。Container没有实现IEnumerator,而是通过实现IEnumerable来提供IEnumerator。请仔细体会这种设计带来的灵活性。
Sample:
string s = "Hello";
// Because string implements IEnumerable, we can call GetEnumerator():
IEnumerator rator = s.GetEnumerator();
while (rator.MoveNext())
{
char c = (char) rator.Current;
Console.Write (c + ".");
}
// Output: H.e.l.l.o.
使用foreach语句改写成:
string s = "Hello"; // The String class implements IEnumerable
foreach (char c in s)
Console.Write (c + ".");
Java:
public class SimpleIteration {
public static void main(String[] args) {
List<Pet> pets = Pets.arrayList(12);
Iterator<Pet> it = pets.iterator();
while(it.hasNext()) {
Pet p = it.next();
System.out.print(p.id() + ":" + p + " ");
}
System.out.println();
// A simpler approach, when possible:
for(Pet p : pets)
System.out.print(p.id() + ":" + p + " ");
System.out.println();
// An Iterator can also remove elements:
it = pets.iterator();
for(int i = 0; i < 6; i++) {
it.next();
it.remove();
}
System.out.println(pets);
} }