Concepts of Programming Languages

11.4 Language Examples 485

with Stack_Pack;

use Stack_Pack;

procedure Use_Stacks is

Topone : Integer;

Stack : Stack_Type; -- Creates a Stack_Type object

begin

Push(Stack, 42);

Push(Stack, 17);

Topone := Top(Stack);

Pop(Stack);

...
end Use_Stacks;

A stack is a silly example for most contemporary languages, because sup-

port for stacks is included in their standard class libraries. However, stacks

provide a simple example we can use to allow comparisons of the languages

discussed in this section.

11.4.1.4 Evaluation

Ada, along with Modula-2, was the first commercial language to support

abstract data types.^2 Although Ada’s design of abstract data types may seem

complicated and repetitious, it clearly provides what is necessary.

11.4.2 Abstract Data Types in C++

C++, which was first released in 1985, was created by adding features to C. The

first important additions were those to support object-oriented programming.

Because one of the primary components of object-oriented programming is

abstract data types, C++ obviously is required to support them.

While Ada provides an encapsulation that can be used to simulate abstract

data types, C++ provides two constructs that are very similar to each other, the

class and the struct, which more directly support abstract data types. Because

structs are most commonly used when only data is included, we do not discuss

them further here.

C++ classes are types; as stated previously, Ada packages are more gen-

eralized encapsulations that can define any number of types. A program unit

that gains visibility to an Ada package can access any of its public entities

directly by their names. A C++ program unit that declares an instance of a

class can also access any of the public entities in that class, but only through

an instance of the class. This is a cleaner and more direct way to provide

abstract data types.

2. The language CLU, which was an academic research language, rather than a commercial
   language, was the first to support abstract data types.