What are Foundation classes, explain?

5 classes:
1. Sets: A set is a container class that may hold any number of distinct values. The order in which the elements occur during iteration is not defined by the sequence in which the elements are inserted.
*** Adding a value to the set which already exists is ignored and does not increase the no. of elements in the set.
2. Map: Map is a data type that associated one (key) value to another value. Both the key and value may be of any valid x++ types.
3. Lists: They are named as Sequential storage of data (Duplicated allowed)
      Lists are the structures that may contain any number of elements that are accessed sequentially. The traversal of the list element is very fast.
4. Arrays: Array is a collection of variables all of the same type, and the variable holds one value at a given time.
      X++ supports only one-dimensional arrays, which have 3 types:
      DYNAMIC - int I [].
      FIXED LENGTH - real r [100] and
      PARTLY ON DISK – date d [, 10] dynamic array of dates with only 10 elements in memory. 10 is a memory element.
                          NoYes e [100, 10] fixed length array, with 10 elements in memory. Value 10 is an optional and if specified, only memory items are stored in memory at once and the rest are stored on disk. These disk items are automatically loaded when referenced, here x++ swaps automatically.
5. Struts: Structures are the entities that may hold any number if values of any x++ types. Struts are used to group info pertaining to a specific entity