The Design and Implementation of the SELF Compiler, an Optimizing Compilet for Object-Oriented Programming Languages

data types [LZ74, LSAS77, LAB+81, LG86] provide a more expressive mechanism for describing and manipulating data structures. An abstract data type abstracts away from a concrete data type by providing a set of operations (the interface) through which clients are to manipulate objects of the type. The abstract data type is implemented in terms of some lower-level data type (the representation), but this implementation is hidden from clients behind the abstract data type’s abstraction boundary. For example, a canonical abstract data type is the stack data type, supporting create, push, pop, top, and isEmpty operations and represented using an array of stack elements and an integer top-of-stack index. The enforced abstraction boundary provides advantages to both implementors and clients of abstract data types over traditional concrete data types. Implementors are free to change the representation of an abstract data type, and as long as the interface remains the same, clients of the abstract data type remain unaffected. For example, the stack data type could be reimplemented using a linked list in place of an array and an integer, and clients would be unaffected. Thus, abstract data types encapsulate design decisions that may change, especially those about the representation of critical data structures. For clients, abstract data types provide a more natural interface for manipulating data structures than the language primitives used with concrete data structures. The operations on abstract data types can directly reflect the conceptual operations on the data type the programmer has in mind, rather than being translated into series of extraction and indexing operations. In the stack example, clients may use the more natural push and pop operations in place of array indexes and integer increments. These abstract operations also improve the reliability of the system, since adding a stack abstract data type