E.2.2 Remote Types Library Units
[A remote types library unit supports the definition
of types intended for use in communication between active partitions.]
Language Design Principles
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
{categorization
pragma (Remote_Types) [partial]} {pragma,
categorization (Remote_Types) [partial]} The
form of a
pragma
Remote_Types is as follows:
pragma Remote_Types[(
library_unit_name)];
Legality Rules
{remote
types library unit} A
remote types
library unit is a library unit to which the pragma Remote_Types applies.
The following restrictions apply to the declaration of such a library
unit:
[it shall be preelaborable;]
it shall depend semantically only on declared pure,
shared passive, or other remote types library units;
it shall not contain the declaration of any variable
within the visible part of the library unit;
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.
{
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).
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.
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.
{
8652/0082}
{
AI95-00164-01}
{remote access type} An
access type declared in the visible part of a remote types or remote
call interface library unit is called a
remote access type.
{remote
access-to-subprogram type} {remote
access-to-class-wide type} Such a type
shall be:
{
8652/0082}
{
AI95-00164-01}
a general access type that designates a class-wide limited private type
or a class-wide private type extension all of whose ancestors are either
private type extensions or limited private types.
The following restrictions
apply to the use of a remote access-to-subprogram type:
{
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;
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.
The following restrictions
apply to the use of a remote access-to-class-wide type:
{
8652/0083}
{
AI95-00047-01}
{
AI95-00240-01}
{
AI95-00366-01}
The primitive subprograms of the corresponding specific limited private
type shall only have access parameters if they are controlling formal
parameters; each non-controlling formal parameter shall support external
streaming (see
13.13.2);
A value of a remote access-to-class-wide type shall
be explicitly converted only to another remote access-to-class-wide type;
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 where the value designates a controlling
operand of the call (see
E.4, “
Remote
Subprogram Calls”).
{
AI95-00366-01}
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.
Reason: All three 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.
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.
Incompatibilities With Ada 95
{
AI95-00240-01}
{
incompatibilities with Ada 95}
Amendment
Correction: The wording was changed from “user-specified”
to “available” 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
{
AI95-00366-01}
{
extensions to Ada 95}
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 signficant incompatibility, as Limited_Controlled
would need stream attributes, and then all extensions of it also would
need stream attributes).
Wording Changes from Ada 95
{
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.
{
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.
{
8652/0082}
{
AI95-00164-01}
Corrigendum: Added missing wording so that a remote access type
can designate an appropriate private extension.
{
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.
{
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.
{
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).
Sponsored by Ada-Europe