Discussion of OOP in Ada

Review of Variant Programming

• Example: Transaction Powerpoints
• Create type hierarchy without inheritance
• Discriminant in record
• Case statement in record field options
• Case statement in client determines code options
• Compile time correctness checks
• Run time correctness checks
• Centralized code
• Easy to add ..., harder to add ...
• Similar to C unions
• Share storate locations

OO Characteristics

• State, Behavior, Identity
• OO is easy as PIE
• Abstraction

Type Derivation

• type height is new float;
• Can add routines, but not fields

Tagged Types

• Example: Transaction Powerpoints
• Allow inheritance (ie adding fields and routines)
• Allow overriding
• Distinguish between type Transaction and Transaction'Class
• Example (from Barnes: Safe and Secure Software):

    package Geometry is
        type Shape is abstract tagged record -- ignoring private for simplicity
            x, y: float;
        end record;
        function area(s: Shape) return float is abstract;
    end Geometry;

    -- Child package for Circles
    package Geometry.Circles is
        type Circle is new Shape with  record
            radius: Float;
        end record;
        function area(c: Circle) return float;  -- Overrides
    end Geometry;

    package body Geometry.Circles is
        function area(c: Circle) return float is
            (3.14159 * c.radius ** 2);
    end Geometry.Circles;
    

• Code:
• geometry.ads and prettified
• geometry-circles.ads and prettified
• geometry-circles.adb and prettified
• geometry-squares.ads and prettified
• geometry-squares.adb and prettified
• ooclient.adb and prettified

Dispatch table

• Seem powerpoints

Overriding

• Potential error with overriding functions:
• Misspelled overriding routine (eg aera)
• Incorrect parameters (eg integer instead of float)


• Considered to be new routine instead of overriding


• How to detect:
• Keyword: overriding


• Not required


• Compiler warning: -gnatyO : warn about not marked overriding routine

Avoiding heap allocation

• Array of pointers to circles