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13.1 Representation Items

There are three kinds of representation items: representation_clauses, component_clauses, and representation pragmas. Representation items specify how the types and other entities of the language are to be mapped onto the underlying machine. 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). Representation items also specify other specifiable properties of entities. 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

   
       representation_clause ::= attribute_definition_clause
             | enumeration_representation_clause
             | record_representation_clause
             | at_clause

       local_name ::= direct_name
             | direct_name'attribute_designator
             | library_unit_name

A representation pragma is allowed only at places where a representation_clause or compilation_unit is allowed.

Name Resolution Rules

In a 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 representation 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 representation 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 a representation_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 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). 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. This includes some padding bits, when the size of the object is greater than the size of its subtype. Such padding bits are considered to be part of the representation of the object, rather than being gaps between objects, if these bits are normally read and updated.

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.

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.

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.

Representation aspects of a generic formal parameter are the same as those of the actual. 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 item that is not supported by the implementation is illegal, or raises an exception at run time.

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 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 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.

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 aspect of representation of an entity is not specified, it is chosen by default in an unspecified manner.

Dynamic Semantics

For the elaboration of a representation_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 aliased component, or a component whose type is by-reference, should always be allocated at an addressable location.

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