Abstract Data Types in Ada

• One of the first languages to offer full support for ADTs

• Package: Syntactic encapsulations for any number of ADTs (unlike C++, Java, C#)
• Package = Specification package + Body package
  • Both have the same name
  • The two are separately compilable
    • Specification package must be compiled first
• Specification package: interface
  • Can hide information in specification package from clients in one of three ways:
    • Declare it in private section of specification package
      • Assignment and comparison operations provided as built-in functions for these types
      • Other functions for the types must be declared in specification package
    • Define the ADT as a pointer to a structure defined entirely in body package
      • Disadvantage: Comparison and assignment is among pointers, not pointees
    • Declare it in limited private section
      • No built-in operations provided - assignment, comparison not meaningful (e.g., for stacks)
      • Cannot (incorrectly) compare/assign pointer data members
• Body package - Optional

• Client
  • Use with clause to make the package visible
  • Use use clause to avoid fully-qualified names
  • Access functions by names, not w.r.t. variables

          with Stack;
          use Stack;
          procedure Client is
          MyStack: StackType;
          begin
             Push( MyStack, 13 );   // Not MyStack.Push( 13 );
          end Client;
          

• Parameterized ADT:
  • Can create generic classes
    • Different source-code versions built by programmer

         generic
            type Element_Type is private:
         package Integer_Stack is new Generic_Stack( Integer );