Operational and Representation Items

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

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.

Syntax

Name Resolution Rules

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.

Legality Rules

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 an aspect of representation 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 type of the subtype denoted by the local_name. The local_name of an operational item shall denote a first subtype. An operational item that names a subtype is type-related.

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

An operational item that directly specifies an aspect of a type shall appear before the type is frozen (see 13.14). If an operational item is given that directly specifies an aspect of a type, then it is illegal to give another operational item that directly specifies the same aspect of the type.

For an untagged derived type, no type-related representation items are allowed if the parent type is a by-reference type, or has any user-defined primitive subprograms.

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. A type-related representation item is not allowed for a descendant of a generic formal untagged type.

A representation item that specifies the Size 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.

A representation or operational item that is not supported by the implementation is illegal, or raises an exception at run time.

A type_declaration is illegal if it has one or more progenitors, and a representation item applies to an ancestor, and this representation item conflicts with the representation of some other ancestor. The cases that cause conflicts are implementation defined.

Static Semantics

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

A derived type inherits each type-related aspect of representation 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 aspect of representation 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 aspect of representation is overridden by a subsequent representation item that specifies the same aspect of the type or subtype.

In contrast, whether operational aspects are inherited by an untagged derived type depends on each specific aspect. Operational aspects are never inherited for a tagged type. When operational aspects are inherited by an untagged derived type, aspects that were directly specified by 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 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 the aspect is specified for the entity, meaning that it is either directly specified or inherited, then that aspect of the entity is as specified, except in the case of Storage_Size, which specifies a minimum.

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.

A representation item that specifies an aspect of representation that would have been chosen in the absence of the representation item is said to be confirming.

Dynamic Semantics

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

Implementation Permissions

An implementation may interpret aspects of representation in an implementation-defined manner. An implementation may place implementation-defined restrictions on representation items. A recommended level of support is specified for representation items 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”).

Implementation Advice

The recommended level of support for all representation items is qualified as follows:

An implementation need not support representation items containing nonstatic expressions, except that an implementation should support a representation item for a given entity if each nonstatic expression in the representation item is a name that statically denotes a constant declared before the entity.

An implementation need not support a specification for the Size for a given composite subtype, nor the size or storage place for an object (including a component) of a given composite subtype, unless the constraints on the subtype and its composite subcomponents (if any) are all static constraints.

An implementation need not support a nonconfirming representation item if it could cause an aliased object or an object of a by-reference type to be allocated at a nonaddressable location or, when the alignment attribute of the subtype of such an object is nonzero, at an address that is not an integral multiple of that alignment.

An implementation need not support a nonconfirming representation item if it could cause an aliased object of an elementary type to have a size other than that which would have been chosen by default.

An implementation need not support a nonconfirming representation item if it could cause an aliased object of a composite type, or an object whose type is by-reference, to have a size smaller than that which would have been chosen by default.

An implementation need not support a nonconfirming subtype-specific representation item specifying an aspect of representation of an indefinite or abstract subtype.

For purposes of these rules, the determination of whether a representation item applied to 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.