13.1 Operational and Representation Aspects

  Two kinds of aspects of entities can be specified: representation aspects and operational aspects. Representation aspects affect how the types and other entities of the language are to be mapped onto the underlying machine. Operational aspects determine other properties of entities.

  Either kind of aspect of an entity may be specified by means of an aspect_specification (see 13.1.1), which is an optional element of most kinds of declarations and applies to the entity or entities being declared. Aspects may also be specified by certain other constructs occurring subsequent to the declaration of the affected entity: a representation aspect value may be specified by means of a representation item and an operational aspect value may be specified by means of an operational item.

There are six kinds of representation items: attribute_definition_clauses for representation attributes, enumeration_representation_clauses, record_representation_clauses, at_clauses, component_clauses, and representation pragmas. They can be provided to give more efficient representation or to interface with features that are outside the domain of the language (for example, peripheral hardware). 

  An operational item is an attribute_definition_clause for an operational attribute.

  An operational item or a representation item applies to an entity identified by a local_name, which denotes an entity declared local to the current declarative region, or a library unit declared immediately preceding a representation pragma in a compilation.

In an operational item or representation item, if the local_name is a direct_name, then it shall resolve to denote a declaration (or, in the case of a pragma, one or more declarations) that occurs immediately within the same declarative region as the item. If the local_name has an attribute_designator, then it shall resolve to denote an implementation-defined component (see 13.5.1) or a class-wide type implicitly declared immediately within the same declarative region as the item. A local_name that is a library_unit_name (only permitted in a representation pragma) shall resolve to denote the library_item that immediately precedes (except for other pragmas) the representation pragma. 

The local_name of an aspect_clause or representation pragma shall statically denote an entity (or, in the case of a pragma, one or more entities) declared immediately preceding it in a compilation, or within the same declarative_part, package_specification, task_definition, protected_definition, or record_definition as the representation or operational item. If a local_name denotes a local callable entity, it may do so through a local subprogram_renaming_declaration (as a way to resolve ambiguity in the presence of overloading); otherwise, the local_name shall not denote a renaming_declaration.

The representation of an object consists of a certain number of bits (the size of the object). For an object of an elementary type, these are the bits that are normally read or updated by the machine code when loading, storing, or operating-on the value of the object. For an object of a composite type, these are the bits reserved for this object, and include bits occupied by subcomponents of the object. If the size of an object is greater than that of its subtype, the additional bits are padding bits. For an elementary object, these padding bits are normally read and updated along with the others. For a composite object, padding bits might not be read or updated in any given composite operation, depending on the implementation.

A representation item directly specifies a representation aspect of the entity denoted by the local_name, except in the case of a type-related representation item, whose local_name shall denote a first subtype, and which directly specifies an aspect of the subtype's type. A representation item that names a subtype is either subtype-specific (Size and Alignment clauses) or type-related (all others). Subtype-specific aspects may differ for different subtypes of the same type.

  An operational item directly specifies an operational aspect of the entity denoted by the local_name, except in the case of a type-related operational item, whose local_name shall denote a first subtype, and which directly specifies an aspect of the type of the subtype.

A representation item that directly specifies an aspect of a subtype or type shall appear after the type is completely defined (see 3.11.1), and before the subtype or type is frozen (see 13.14). 

  An operational item that directly specifies an aspect of an entity shall appear before the entity is frozen (see 13.14).

  If a representation item, operational item, or aspect_specification is given that directly specifies an aspect of an entity, then it is illegal to give another representation item, operational item, or aspect_specification that directly specifies the same aspect of the entity.

  Unless otherwise specified, it is illegal to specify an operational or representation aspect of a generic formal parameter.

 For an untagged derived type, it is illegal to specify a type-related representation aspect if the parent type is a by-reference type, or has any user-defined primitive subprograms. Similarly, it is illegal to specify a nonconfirming type-related representation aspect for an untagged by-reference type after one or more types have been derived from it. 

 Operational and representation aspects of a generic formal parameter are the same as those of the actual. Operational and representation aspects are the same for all views of a type. Specification of a type-related representation aspect is not allowed for a descendant of a generic formal untagged type. 

 The specification of the Size aspect for a given subtype, or the size or storage place for an object (including a component) of a given subtype, shall allow for enough storage space to accommodate any value of the subtype.

 If a specification of a representation or operational aspect is not supported by the implementation, it is illegal or raises an exception at run time.

   A type_declaration is illegal if it has one or more progenitors, and a nonconfirming value was specified for a representation aspect of an ancestor, and this conflicts with the representation of some other ancestor. The cases that cause conflicts are implementation defined. 

If two subtypes statically match, then their subtype-specific aspects (Size and Alignment) are the same.

 A derived type inherits each type-related representation aspect of its parent type that was directly specified before the declaration of the derived type, or (in the case where the parent is derived) that was inherited by the parent type from the grandparent type. A derived subtype inherits each subtype-specific representation aspect of its parent subtype that was directly specified before the declaration of the derived type, or (in the case where the parent is derived) that was inherited by the parent subtype from the grandparent subtype, but only if the parent subtype statically matches the first subtype of the parent type. An inherited representation aspect is overridden by a subsequent aspect_specification or representation item that specifies a different value for the same aspect of the type or subtype. 

   In contrast, whether operational aspects are inherited by a derived type depends on each specific aspect; unless specified, an operational aspect is not inherited. When operational aspects are inherited by a derived type, aspects that were directly specified by aspect_specifications or operational items that are visible at the point of the derived type declaration, or (in the case where the parent is derived) that were inherited by the parent type from the grandparent type are inherited. An inherited operational aspect is overridden by a subsequent aspect_specification or operational item that specifies the same aspect of the type. 

   When an aspect that is a subprogram is inherited, the derived type inherits the aspect in the same way that a derived type inherits a user-defined primitive subprogram from its parent (see 3.4).

Each aspect of representation of an entity is as follows: 

   If an operational aspect is specified for an entity (meaning that it is either directly specified or inherited), then that aspect of the entity is as specified. Otherwise, the aspect of the entity has the default value for that aspect.

   An aspect_specification or representation item that specifies a representation aspect that would have been chosen in the absence of the aspect_specification or representation item is said to be confirming. The aspect value specified in this case is said to be a confirming representation aspect value. Other values of the aspect are said to be nonconfirming, as are the aspect_specifications and representation items that specified them.

 For the elaboration of an aspect_clause, any evaluable constructs within it are evaluated. 

 An implementation may interpret representation aspects in an implementation-defined manner. An implementation may place implementation-defined restrictions on the specification of representation aspects. A recommended level of support is defined for the specification of representation aspects and related features in each subclause. These recommendations are changed to requirements for implementations that support the Systems Programming Annex (see C.2, “Required Representation Support”).

 The recommended level of support for the specification of all representation aspects is qualified as follows: 

 For purposes of these rules, the determination of whether specifying a representation aspect value for a type could cause an object to have some property is based solely on the properties of the type itself, not on any available information about how the type is used. In particular, it presumes that minimally aligned objects of this type might be declared at some point.