3.2 Types and Subtypes

1. A type is characterized by a set of values, and a set of primitive operations which implement the fundamental aspects of its semantics. An object of a given type is a run-time entity that contains (has) a value of the type.
2. Types are grouped into classes of types, reflecting the similarity of their values and primitive operations. There exist several language-defined classes of types (see NOTES below). Elementary types are those whose values are logically indivisible; composite types are those whose values are composed of component values.
3. The elementary types are the scalar types (discrete and real) and the access types (whose values provide access to objects or subprograms). Discrete types are either integer types or are defined by enumeration of their values (enumeration types). Real types are either floating point types or fixed point types.
4. The composite types are the record types, record extensions, array types, task types, and protected types. A private type or private extension represents a partial view, See section 7.3 Private Types and Private Extensions, of a type, providing support for data abstraction. A partial view is a composite type.
5. Certain composite types (and partial views thereof) have special components called discriminants whose values affect the presence, constraints, or initialization of other components. Discriminants can be thought of as parameters of the type.
6. The term subcomponent is used in this International Standard in place of the term component to indicate either a component, or a component of another subcomponent. Where other subcomponents are excluded, the term component is used instead. Similarly, a part of an object or value is used to mean the whole object or value, or any set of its subcomponents.
7. The set of possible values for an object of a given type can be subjected to a condition that is called a constraint (the case of a null constraint that specifies no restriction is also included); the rules for which values satisfy a given kind of constraint are given in See section 3.5 Scalar Types for range_constraints, See section 3.6.1 Index Constraints and Discrete Ranges, for index_constraints, and See section 3.7.1 Discriminant Constraints, for discriminant_constraints.
8. A subtype of a given type is a combination of the type, a constraint on values of the type, and certain attributes specific to the subtype. The given type is called the type of the subtype. Similarly, the associated constraint is called the constraint of the subtype. The set of values of a subtype consists of the values of its type that satisfy its constraint. Such values belong to the subtype.
9. A subtype is called an unconstrained subtype if its type has unknown discriminants, or if its type allows range, index, or discriminant constraints, but the subtype does not impose such a constraint; otherwise, the subtype is called a constrained subtype (since it has no unconstrained characteristics).
   

   NOTES
   

10. (2) Any set of types that is closed under derivation See section 3.4 Derived Types and Classes, can be called a "class" of types. However, only certain classes are used in the description of the rules of the language -- generally those that have their own particular set of primitive operations See section 3.2.3 Classification of Operations, or that correspond to a set of types that are matched by a given kind of generic formal type See section 12.5 Formal Types. The following are examples of "interesting" language-defined classes: elementary, scalar, discrete, enumeration, character, boolean, integer, signed integer, modular, real, floating point, fixed point, ordinary fixed point, decimal fixed point, numeric, access, access-to-object, access-to-subprogram, composite, array, string, (untagged) record, tagged, task, protected, nonlimited. Special syntax is provided to define types in each of these classes.
    1. These language-defined classes are organized like this:

    2. all types
          elementary
             scalar
                discrete
                   enumeration
                      character
                      boolean
                      other enumeration
                   integer
                      signed integer
                      modular integer
                real
                   floating point
                   fixed point
                      ordinary fixed point
                      decimal fixed point
             access
                access-to-object
                access-to-subprogram
          composite
             array
                string
                other array
             untagged record
             tagged
             task
             protected
       

    3. The classes "numeric" and "nonlimited" represent other classification dimensions and do not fit into the above strictly hierarchical picture.

• 3.2.1: Type Declarations
• 3.2.2: Subtype Declarations
• 3.2.3: Classification of Operations

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