Chapter 5: Programming Practices - TOC - 5.3 TYPES

5.3.2 Anonymous Types

guideline

Avoid anonymous array types.

Use anonymous array types for array variables only when no suitable type exists or can be created and the array will not be referenced as a whole (e.g., used as a subprogram parameter).

Use access parameters and access discriminants to guarantee that the parameter or discriminant is treated as a constant.

example

Use:

type Buffer_Index is range 1 .. 80;
type Buffer       is array (Buffer_Index) of Character;
Input_Line : Buffer;

rather than:

Input_Line : array (Buffer_Index) of Character;

rationale

Although Ada allows anonymous types, they have limited usefulness and complicate program modification. For example, except for arrays, a variable of anonymous type can never be used as an actual parameter because it is not possible to define a formal parameter of the same type. Even though this may not be a limitation initially, it precludes a modification in which a piece of code is changed to a subprogram. Although you can declare the anonymous array to be aliased, you cannot use this access value as an actual parameter in a subprogram because the subprogram's formal parameter declaration requires a type mark. Also, two variables declared using the same anonymous type declaration are actually of different types.

Even though the implicit conversion of array types during parameter passing is supported in Ada, it is difficult to justify not using the type of the parameter. In most situations, the type of the parameter is visible and easily substituted in place of an anonymous array type. The use of an anonymous array type implies that the array is only being used as a convenient way to implement a collection of values. It is misleading to use an anonymous type, and then treat the variable as an object.

When you use an access parameter or access discriminant, the anonymous type is essentially declared inside the subprogram or object itself (Rationale 1995, §3.7.1). Thus, you have no way of declaring another object of the same type, and the object is treated as a constant. In the case of a self-referential data structure (see Guideline 5.4.6), you need the access parameter to be able to manipulate the data the discriminant accesses (Rationale 1995, §3.7.1).

notes

For anonymous task types, see Guideline 6.1.4.

exceptions

If you are creating a unique table, for example, the periodic table of the elements, consider using an anonymous array type.