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E.2.2 Remote Types Library Units

1
[A remote types library unit supports the definition of types intended for use in communication between active partitions.]

Language Design Principles

1.a
The restrictions governing a remote types package are similar to those for a declared pure package. However, the restrictions are relaxed deliberately to allow such a package to contain declarations that violate the stateless property of pure packages, though it is presumed that any state-dependent properties are essentially invisible outside the package. 

Syntax

2
The form of a pragma Remote_Types is as follows: 
3
  pragma Remote_Types[(library_unit_name)];

Legality Rules

4/3
{AI05-0243-1} A pragma Remote_Types is used to specify that a library unit is a remote types library unit, namely that the Remote_Types aspect of the library unit is True. The following restrictions apply to the declaration of such a library unit:
4.a/3
Aspect Description for Remote_Types: Types in a given package may be used in remote procedure calls.
5
[it shall be preelaborable;]
6/3
{AI05-0206-1} {AI05-0243-1} it shall depend semantically only on declared pure library_items, shared passive library units, other remote types library units, or preelaborated normal library units that are mentioned only in private with clauses;
6.a/3
Ramification: {AI05-0243-1} We say declared pure library_item here, so that (all) limited views are allowed; those are not library units, but they are declared pure library_items.
7
it shall not contain the declaration of any variable within the visible part of the library unit; 
7.a
Reason: This is essentially a “methodological” restriction. A separate copy of a remote types package is included in each partition that references it, just like a normal package. Nevertheless, a remote types package is thought of as an “essentially pure” package for defining types to be used for interpartition communication, and it could be misleading to declare visible objects when no remote data access is actually being provided. 
8/2
{AI95-00240-01} {AI95-00366-01} the full view of each type declared in the visible part of the library unit that has any available stream attributes shall support external streaming (see 13.13.2). 
8.a
Reason: This is to prevent the use of the predefined Read and Write attributes of an access type as part of the Read and Write attributes of a visible type. 
8.b/2
Ramification: {AI95-00366-01} Types that do not have available stream attributes are excluded from this rule; that means that attributes do not need to be specified for most limited types. It is only necessary to specify attributes for nonlimited types that have a part that is of any access type, and for extensions of limited types with available stream attributes where the record_extension_part includes a subcomponent of an access type, where the access type does not have specified attributes. 
9/3
{8652/0082} {AI95-00164-01} {AI05-0060-1} A named access type declared in the visible part of a remote types or remote call interface library unit is called a remote access type. Such a type shall be:
9.1/1
{8652/0082} {AI95-00164-01} an access-to-subprogram type, or
9.2/3
{8652/0082} {AI95-00164-01} {AI05-0060-1} a general access type that designates a class-wide limited private type, a class-wide limited interface type, or a class-wide private extension all of whose ancestors are either private extensions, limited interface types, or limited private types. 
9.3/1
  {8652/0081} {AI95-00004-01} A type that is derived from a remote access type is also a remote access type.
10
The following restrictions apply to the use of a remote access-to-subprogram type: 
11/2
{AI95-00431-01} A value of a remote access-to-subprogram type shall be converted only to or from another (subtype-conformant) remote access-to-subprogram type;
12
The prefix of an Access attribute_reference that yields a value of a remote access-to-subprogram type shall statically denote a (subtype-conformant) remote subprogram. 
13
The following restrictions apply to the use of a remote access-to-class-wide type: 
14/3
{8652/0083} {AI95-00047-01} {AI95-00240-01} {AI95-00366-01} {AI05-0060-1} {AI05-0101-1} The primitive subprograms of the corresponding specific type shall only have access parameters if they are controlling formal parameters. The primitive functions of the corresponding specific type shall only have an access result if it is a controlling access result. Each noncontrolling formal parameter and noncontrolling result type shall support external streaming (see 13.13.2);
14.1/3
{AI05-0060-1} {AI05-0215-1} {AI05-0269-1} The corresponding specific type shall not have a primitive procedure with the Synchronization aspect specified unless the synchronization_kind is Optional (see 9.5);
15
A value of a remote access-to-class-wide type shall be explicitly converted only to another remote access-to-class-wide type;
16/4
{AI12-0034-1} A value of a remote access-to-class-wide type shall be dereferenced (or implicitly converted to an anonymous access type) only as part of a dispatching call to a primitive operation of the designated type where the value designates a controlling operand of the call (see E.4, “Remote Subprogram Calls”); 
16.a/4
Ramification: {AI12-0034-1} Stream attributes of the designated type are not primitive operations of the designated type, and thus remote calls to them are prohibited by this rule. This is good, as the access parameter of a stream attribute does not have external streaming, and thus cannot be a parameter of a remote call. 
16.1/3
{AI05-0101-1} A controlling access result value for a primitive function with any controlling operands of the corresponding specific type shall either be explicitly converted to a remote access-to-class-wide type or be part of a dispatching call where the value designates a controlling operand of the call;
17/4
{AI95-00366-01} {AI12-0085-1} The Storage_Pool attribute is not defined for a remote access-to-class-wide type; the expected type for an allocator shall not be a remote access-to-class-wide type. A remote access-to-class-wide type shall not be an actual parameter for a generic formal access type. The Storage_Size attribute of a remote access-to-class-wide type yields 0; it is not allowed in an attribute_definition_clause.  The Storage_Pool and Storage_Size aspects shall not be specified for a remote access-to-class-wide type. 
17.a/3
Reason: {AI05-0005-1} All of these restrictions are because there is no storage pool associated with a remote access-to-class-wide type. The Storage_Size is defined to be 0 so that there is no conflict with the rules for pure units. 
17.b/4
Ramification: {AI12-0085-1} The prohibition against specifying the Storage_Size aspect for an access-to-class-wide type applies to any method of doing that, including via either a aspect_specification or an attribute_definition_clause.

Erroneous Execution

17.1/4
   {AI12-0076-1} Execution is erroneous if some operation (other than the initialization or finalization of the object) modifies the value of a constant object declared in the visible part of a remote types package.
17.c/4
Discussion: This could be accomplished via a self-referencing pointer or via squirrelling a writable pointer to a controlled object. 
NOTES
18
5  A remote types library unit need not be pure, and the types it defines may include levels of indirection implemented by using access types. User-specified Read and Write attributes (see 13.13.2) provide for sending values of such a type between active partitions, with Write marshalling the representation, and Read unmarshalling any levels of indirection.
19/3
6  {AI05-0060-1} The value of a remote access-to-class-wide limited interface can designate an object of a nonlimited type derived from the interface.
20/3
7  {AI05-0060-1} A remote access type may designate a class-wide synchronized, protected, or task interface type. 
20.a/3
Proof: Synchronized, protected, and task interfaces are all considered limited interfaces, see 3.9.4.

Incompatibilities With Ada 95

20.b/3
{AI95-00240-01} {AI05-0248-1} Amendment Correction: The wording was changed from “user-specified” to “available” read and write attributes. (This was then further changed, see below.) This means that an access type with the attributes specified in the private part would originally have been sufficient to allow the access type to be used in a remote type, but that is no longer allowed. Similarly, the attributes of a remote type that has access components have to be specified in the visible part. These changes were made so that the rules were consistent with the rules introduced for the Corrigendum for stream attributes; moreover, legality should not depend on the contents of the private part.

Extensions to Ada 95

20.c/3
{AI95-00366-01} {AI05-0005-1} Remote types that cannot be streamed (that is, have no available stream attributes) do not require the specification of stream attributes. This is necessary so that most extensions of Limited_Controlled do not need stream attributes defined (otherwise there would be a significant incompatibility, as Limited_Controlled would need stream attributes, and then all extensions of it also would need stream attributes). 

Wording Changes from Ada 95

20.d/2
{8652/0081} {AI95-00004-01} Corrigendum: Added missing wording so that a type derived from a remote access type is also a remote access type.
20.e/2
{8652/0083} {AI95-00047-01} Corrigendum: Clarified that user-defined Read and Write attributes are required for the primitive subprograms corresponding to a remote access-to-class-wide type.
20.f/2
{8652/0082} {AI95-00164-01} Corrigendum: Added missing wording so that a remote access type can designate an appropriate private extension.
20.g/2
{AI95-00366-01} Changed the wording to use the newly defined term type that supports external streaming, so that various issues with access types in pure units and implicitly declared attributes for type extensions are properly handled.
20.h/2
{AI95-00366-01} Defined Storage_Size to be 0 for remote access-to-class-wide types, rather than having it undefined. This eliminates issues with pure units requiring a defined storage size.
20.i/2
{AI95-00431-01} Corrected the wording so that a value of a local access-to-subprogram type cannot be converted to a remote access-to-subprogram type, as intended (and required by the ACATS). 

Incompatibilities With Ada 2005

20.j/4
{AI05-0101-1} {AI12-0005-1} Correction: Added rules for returning of remote access-to-class-wide access-to-classwide types; this had been missed in the past. While programs that returned unstreamable types from RCI functions were legal, it is not clear what they could have done (as the results could not be marshalled). Similarly, RCI functions that return remote controlling access types could try to save those values, but it is unlikely that a compiler would know how to do that usefully. Thus, it seems unlikely that any real programs will be impacted by these changes. 

Extensions to Ada 2005

20.k/3
{AI05-0060-1} Correction: Clarified that anonymous access types are never remote access types (and can be used in remote types units subject to the normal restrictions). Added wording to allow limited class-wide interfaces to be designated by remote access types.
20.l/3
{AI05-0206-1} Added wording to allow private withs of preelaborated normal units in the specification of a remote types unit.
20.m/3
{AI05-0243-1} Remote_Types is now a categorization aspect, so it can be specified by an aspect_specification — although the pragma is still preferred by the Standard. 

Wording Changes from Ada 2012

20.n/4
{AI12-0034-1} Corrigendum: Clarified that dispatching remote stream attribute calls are prohibited. We don't document this as an incompatibility, as the stream parameter cannot be marshalled for a remote call (it doesn't have external streaming), so it's impossible that any working program depends on this functionality.
20.o/4
{AI12-0076-1} Corrigendum: Explicitly stated that modifying a visible constant in a remote types package is erroneous. We don't document this as inconsistent as implementations certainly can still do whatever they were previously doing (no change is required); moreover, this case (and many more) were erroneous in Ada 2005 and before, so we're just restoring the previous semantics.
20.p/4
{AI12-0085-1} Corrigendum: Clarified that specifying the Storage_Pool or Storage_Size aspect for an access-to-class-wide type is not allowed. The intent is clear, and no implementation has ever allowed specifying the aspects (the attributes already cannot be specified), so we don't document this as an incompatibility. 

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