Chapter 9: Object-Oriented Features - TOC - 9.3 TAGGED TYPE OPERATIONS

9.3.2 Class-Wide Operations

guideline

Consider using a class-wide operation (i.e., an operation with parameter[s] of a class-wide type) when an operation can be written, compiled, and tested without knowing all the possible descendants of a given tagged type (Barnes 1996).

Consider using a class-wide operation when you do not want an operation to be inherited and/or overridden.

example

The following example is adapted from Barnes (1996) using the geometric objects from the example of Guideline 9.2.1 and declaring the following functions as primitives in the package specification:

function Area (O : in Object) return Float;

function Area (C : in Circle) return Float;

function Area (S : in Shape) return Float;

A function for computing the moment of a force about a fulcrum can now be created using a class-wide type as follows:

function Moment (OC : Object'Class) return Float is
begin
   return OC.X_Coord*Area(OC);
end Moment;

Because Moment accepts the class-wide formal parameter of Object'Class, it can be called with an actual parameter that is any derivation of type Object. Assuming that all derivations of type object have defined a function for Area, Moment will dispatch to the appropriate function when called. For example:

C : Circle;
M : Float;

...

-- Moment will dispatch to the Area function for the Circle type.
M := Moment(C);

rationale

The use of class-wide operations avoids unnecessary duplication of code. Run-time dispatching may be used where necessary to invoke appropriate type-specific operations based on an operand's tag.

See also Guideline 8.4.3 for a discussion of class-wide pointers in an object-oriented programming framework registry.