How to define an interator to provide array like behaviour in a class?

[Gary Willoughby]

I want to do something like this (Collection is a custom type):
Collection x = new Collection();
x.add(something);
x.add(somethingElse);

foreach(type value; x)
{
    writeln(value);
}

Collection is a class with a private array member variable which 
actually holds the collection data entered via the add method. 
What do i need to add to the Collection class to make it iterable 
(whilst it's actually iterating through the member array) so the 
foreach loop works as expected?