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9.6 Summary

tagged type hierarchies

  • Consider using type extension when designing an is-a (generalization/specialization) hierarchy.
  • Use tagged types to preserve a common interface across differing implementations (Taft 1995a).
  • When defining a tagged type in a package, consider including a definition of a general access type to the corresponding class-wide type.
  • In general, define only one tagged type per package.
  • The implementation of the dispatching operations of each type in a derivation class rooted in a tagged type T should conform to the expected semantics of the corresponding dispatching operations of the class-wide type T'Class.
  • Consider using a controlled type whenever a type allocates resources that must be deallocated or otherwise "cleaned up" on destruction or overwriting.
  • Use a derivation from a controlled type in preference to providing an explicit "cleanup" operation that must be called by clients of the type.
  • When overriding the adjustment and finalization procedures derived from controlled types, define the finalization procedure to undo the effects of the adjustment procedure.
  • Derived type initialization procedures should call the initialization procedure of their parent as part of their type-specific initialization.
  • Derived type finalization procedures should call the finalization procedure of their parent as part of their type-specific finalization.
  • Consider deriving a data structure's components rather than the enclosing data structure from a controlled type.
  • Consider using abstract types and operations in creating classification schemes, for example, a taxonomy, in which only the leaf objects will be meaningful in the application.
  • Consider declaring root types and internal nodes in a type tree as abstract.
  • Consider using abstract types for generic formal derived types.
  • Consider using abstract types to develop different implementations of a single abstraction.

tagged type operations

  • Consider declaring a primitive abstract operation based on the absence of a meaningful "default" behavior.
  • Consider declaring a primitive nonabstract operation based on the presence of a meaningful "default" behavior.
  • When overriding an operation, the overriding subprogram should not raise exceptions that are not known to the users of the overridden subprogram.
  • If redispatching is used in the implementation of the operations of a type, with the specific intent that some of the redispatched-to operations be overridden by specializations for the derived types, then document this intent clearly in the specification as part of the "interface" of a parent type with its derived types.
  • When redispatching is used (for any reason) in the implementation of a primitive operation of a tagged type, then document (in some project-consistent way) this use in the body of the operation subprogram so that it can be easily found during maintenance.
  • 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.
  • Avoid declaring a constructor as a primitive abstract operation.
  • Use a primitive abstract operation to declare an initialization function or constructor only when objects of the inheriting derived types will not require additional parameters for initialization.
  • Consider using access discriminants to provide parameters to default initialization.
  • Use constructors for explicit initialization.
  • Consider splitting the initialization and construction of an object.
  • Consider declaring a constructor operation in a child package.
  • Consider declaring a constructor operation to return an access value to the constructed object (Dewar 1995).
  • When you redefine the "=" operator on a tagged type, make sure that it has the expected behavior in extensions of this type and override it if necessary.
  • Consider using class-wide programming to provide run-time, dynamic polymorphism when constructing larger, reusable, extensible frameworks.
  • When possible, use class-wide programming rather than variant records.
  • Use class-wide programming to provide a consistent interface across the set of types in the tagged type hierarchy (i.e., class).
  • Consider using generics to define a new type in terms of an existing type, either as an extension or as a container, collection, or composite data structure.
  • Avoid using type extensions for parameterized abstractions when generics provide a more appropriate mechanism.

managing visibility

  • Consider giving derived tagged types the same visibility to the parent type as other clients of the parent.
  • Define a derived tagged type in a child of the package that defines the base type if the implementation of the derived type requires greater visibility into the implementation of the base type than other clients of the base type require.

multiple inheritance

  • Consider using type composition for implementation, as opposed to interface, inheritance.
  • Consider using a generic to "mix in" functionality to a derivative of some core abstraction.
  • Consider using access discriminants to support "full" multiple inheritance where an object must be referenceable as an entity of two or more distinct unrelated abstractions.

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