13.13.2 Stream-Oriented Attributes
{
8652/0009}
{
AI95-00137-01}
The operational attributes Write, Read, Output, and Input convert values
to a stream of elements and reconstruct values from a stream.
Static Semantics
{
AI95-00270-01}
For every subtype S of an elementary type
T, the following representation
attribute is defined:
S'Stream_Size
{
AI95-00270-01}
Denotes the number of bits occupied in a stream by items of subtype S.
Hence, the number of stream elements required per item of elementary
type
T is:
T'Stream_Size / Ada.Streams.Stream_Element'Size
The value of this attribute is of type
universal_integer and is a multiple of Stream_Element'Size.
Stream_Size may be specified for first
subtypes via an
attribute_definition_clause;
the
expression
of such a clause shall be static, nonnegative, and a multiple of Stream_Element'Size.
Discussion: Stream_Size is a type-related
attribute (see
13.1).
Implementation Advice
{
AI95-00270-01}
If not specified, the value of Stream_Size for an elementary type should
be the number of bits that corresponds to the minimum number of stream
elements required by the first subtype of the type, rounded up to the
nearest factor or multiple of the word size that is also a multiple of
the stream element size.
Implementation Advice: If not specified,
the value of Stream_Size for an elementary type should be the number
of bits that corresponds to the minimum number of stream elements required
by the first subtype of the type, rounded up to the nearest factor or
multiple of the word size that is also a multiple of the stream element
size.
Reason: {
AI95-00270-01}
This is Implementation Advice because we want to allow implementations
to remain compatible with their Ada 95 implementations, which may have
a different handling of the number of stream elements. Users can always
specify Stream_Size if they need a specific number of stream elements.
{
AI95-00270-01}
{recommended level of support (Stream_Size
attribute) [partial]} The recommended
level of support for the Stream_Size attribute is:
{
AI95-00270-01}
A Stream_Size clause should be supported for a discrete or fixed point
type
T if the specified Stream_Size is a multiple of Stream_Element'Size
and is no less than the size of the first subtype of
T, and no
greater than the size of the largest type of the same elementary class
(signed integer, modular integer, enumeration, ordinary fixed point,
or decimal fixed point).
Implementation Advice: The recommended
level of support for the Stream_Size attribute should be followed.
Ramification: There are no requirements
beyond supporting confirming Stream_Size clauses for floating point and
access types. Floating point and access types usually only have a handful
of defined formats, streaming anything else makes no sense for them.
For discrete and fixed point types, this may
require support for sizes other than the “natural” ones.
For instance, on a typical machine with 32-bit integers and a Stream_Element'Size
of 8, setting Stream_Size to 24 must be supported. This is required as
such formats can be useful for interoperability with unusual machines,
and there is no difficulty with the implementation (drop extra bits on
output, sign extend on input).
Static Semantics
For every subtype S of a specific type T,
the following attributes are defined.
S'Write
S'Write denotes a procedure with
the following specification:
{
AI95-00441-01}
procedure S'Write(
Stream :
not null access Ada.Streams.Root_Stream_Type'Class;
Item :
in T)
S'Write writes the value of Item
to Stream.
S'Read
S'Read denotes a procedure with
the following specification:
{
AI95-00441-01}
procedure S'Read(
Stream :
not null access Ada.Streams.Root_Stream_Type'Class;
Item :
out T)
S'Read reads the value of Item from
Stream.
{
8652/0040}
{
AI95-00108-01}
{
AI95-00444-01}
For an untagged derived type, the Write (resp. Read) attribute is inherited
according to the rules given in
13.1 if the
attribute is available for the parent type at the point where
T
is declared. For a tagged derived type, these attributes are not inherited,
but rather the default implementations are used.
{
AI95-00444-01}
The default implementations of the Write and Read attributes, where available,
execute as follows:
{
8652/0040}
{
AI95-00108-01}
{
AI95-00195-01}
{
AI95-00251-01}
{
AI95-00270-01}
For elementary types, Read reads (and Write writes) the number of stream
elements implied by the Stream_Size for the type
T; the representation
of those stream elements is implementation defined. For composite types,
the Write or Read attribute for each component is called in canonical
order, which is last dimension varying fastest for an array, and positional
aggregate order for a record. Bounds are not included in the stream if
T is an array type. If
T is a discriminated type, discriminants
are included only if they have defaults. If
T is a tagged type,
the tag is not included. For type extensions, the Write or Read attribute
for the parent type is called, followed by the Write or Read attribute
of each component of the extension part, in canonical order. For a limited
type extension, if the attribute of the parent type or any progenitor
type of
T is available anywhere within the immediate scope of
T, and the attribute of the parent type or the type of any of
the extension components is not available at the freezing point of
T,
then the attribute of
T shall be directly specified.
Implementation defined: The contents
of the stream elements read and written by the Read and Write attributes
of elementary types.
Reason: A discriminant with a default
value is treated simply as a component of the object. On the other hand,
an array bound or a discriminant without a default value, is treated
as “descriptor” or “dope” that must be provided
in order to create the object and thus is logically separate from the
regular components. Such “descriptor” data are written by
'Output and produced as part of the delivered result by the 'Input function,
but they are not written by 'Write nor read by 'Read. A tag is like a
discriminant without a default.
{
8652/0040}
{
AI95-00108-01}
For limited type extensions, we must have a definition of 'Read and 'Write
if the parent type has one, as it is possible to make a dispatching call
through the attributes. The rule is designed to automatically do the
right thing in as many cases as possible.
{
AI95-00251-01}
Similarly, a type that has a progenitor with an available attribute must
also have that attribute, for the same reason.
Ramification: {
AI95-00195-01}
For a composite object, the subprogram denoted by the Write or Read attribute
of each component is called, whether it is the default or is user-specified.
Implementations are allowed to optimize these calls (see below), presuming
the properties of the attributes are preserved.
{
AI95-00270-01}
Constraint_Error is raised by the predefined Write attribute if the value
of the elementary item is outside the range of values representable using
Stream_Size bits. For a signed integer type, an enumeration type, or
a fixed point type, the range is unsigned only if the integer code for
the lower bound of the first subtype is nonnegative, and a (symmetric)
signed range that covers all values of the first subtype would require
more than Stream_Size bits; otherwise the range is signed.
For every subtype S'Class
of a class-wide type T'Class:
S'Class'Write
S'Class'Write denotes a procedure
with the following specification:
{
AI95-00441-01}
procedure S'Class'Write(
Stream :
not null access Ada.Streams.Root_Stream_Type'Class;
Item :
in T'Class)
Dispatches to the subprogram denoted by
the Write attribute of the specific type identified by the tag of Item.
S'Class'Read
S'Class'Read denotes a procedure
with the following specification:
{
AI95-00441-01}
procedure S'Class'Read(
Stream :
not null access Ada.Streams.Root_Stream_Type'Class;
Item :
out T'Class)
Dispatches to the subprogram denoted by
the Read attribute of the specific type identified by the tag of Item.
Reason: It is necessary to have class-wide
versions of Read and Write in order to avoid generic contract model violations;
in a generic, we don't necessarily know at compile time whether a given
type is specific or class-wide.
Implementation Advice
Static Semantics
For every subtype S of a specific type T,
the following attributes are defined.
S'Output
S'Output denotes a procedure
with the following specification:
{
AI95-00441-01}
procedure S'Output(
Stream :
not null access Ada.Streams.Root_Stream_Type'Class;
Item :
in T)
S'Output writes the value of Item
to Stream, including any bounds or discriminants.
Ramification: Note that the bounds are
included even for an array type whose first subtype is constrained.
S'Input
S'Input denotes a function with
the following specification:
{
AI95-00441-01}
function S'Input(
Stream :
not null access Ada.Streams.Root_Stream_Type'Class)
return TS'Input reads and returns one value from
Stream, using any bounds or discriminants written by a corresponding
S'Output to determine how much to read.
{
8652/0040}
{
AI95-00108-01}
{
AI95-00444-01}
For an untagged derived type, the Output (resp. Input) attribute is inherited
according to the rules given in
13.1 if the
attribute is available for the parent type at the point where
T
is declared. For a tagged derived type, these attributes are not inherited,
but rather the default implementations are used.
{
AI95-00444-01}
The default implementations of the Output and Input attributes, where
available, execute as follows:
If T is an array type, S'Output first writes
the bounds, and S'Input first reads the bounds. If T has discriminants
without defaults, S'Output first writes the discriminants (using S'Write
for each), and S'Input first reads the discriminants (using S'Read for
each).
{
AI95-00195-01}
S'Output then calls S'Write to write the value of
Item to the
stream. S'Input then creates an object (with the bounds or discriminants,
if any, taken from the stream), passes it to S'Read, and returns the
value of the object. Normal default initialization and finalization take
place for this object (see
3.3.1,
7.6,
and
7.6.1).
{
AI95-00251-01}
If
T is an abstract type, then S'Input is an abstract function.
Ramification: For an abstract type
T,
S'Input can be called in a dispatching call, or passed to a abstract
formal subprogram. But it cannot be used in non-dispatching contexts,
because we don't allow objects of abstract types to exist. The designation
of this function as abstract has no impact on descendants of
T,
as
T'Input is not inherited for tagged types, but rather recreated
(and the default implementation of
T'Input calls
T'Read,
not the parent type's
T'Input). Note that
T'Input cannot
be specified in this case, as any function with the proper profile is
necessarily abstract, and specifying abstract subprograms in an
attribute_definition_clause
is illegal.
For every subtype S'Class
of a class-wide type T'Class:
S'Class'Output
S'Class'Output denotes a procedure
with the following specification:
{
AI95-00441-01}
procedure S'Class'Output(
Stream :
not null access Ada.Streams.Root_Stream_Type'Class;
Item :
in T'Class)
{
AI95-00344-01}
First writes the external tag of
Item to
Stream (by calling
String'Output(
Stream, Tags.External_Tag(
Item'Tag)) —
see
3.9) and then dispatches to the subprogram
denoted by the Output attribute of the specific type identified by the
tag. Tag_Error is raised if the tag of Item identifies a type declared
at an accessibility level deeper than that of S.
Reason: {
AI95-00344-01}
We raise Tag_Error here for nested types as such a type cannot be successfully
read with S'Class'Input, and it doesn't make sense to allow writing a
value that cannot be read.
S'Class'Input
S'Class'Input denotes a function
with the following specification:
{
AI95-00441-01}
function S'Class'Input(
Stream :
not null access Ada.Streams.Root_Stream_Type'Class)
return T'Class
{
AI95-00279-01}
{
AI95-00344-01}
First reads the external tag from
Stream and determines the corresponding
internal tag (by calling Tags.Descendant_Tag(String'Input(
Stream),
S'Tag) which might raise Tag_Error — see
3.9)
and then dispatches to the subprogram denoted by the Input attribute
of the specific type identified by the internal tag; returns that result.
If the specific type identified by the internal tag is not covered by
T'Class or is abstract, Constraint_Error is raised.
{
AI95-00195-01}
{Range_Check [partial]} {check,
language-defined (Range_Check)} In the
default implementation of Read and Input for a composite type, for each
scalar component that is a discriminant or whose
component_declaration
includes a
default_expression,
a check is made that the value returned by Read for the component belongs
to its subtype.
{Constraint_Error (raised
by failure of run-time check)} Constraint_Error
is raised if this check fails. For other scalar components, no check
is made. For each component that is of an access type, if the implementation
can detect that the value returned by Read for the component is not a
value of its subtype, Constraint_Error is raised. If the value is not
a value of its subtype and this error is not detected, the component
has an abnormal value, and erroneous execution can result (see
13.9.1).
In the default implementation of Read for a composite type with defaulted
discriminants, if the actual parameter of Read is constrained, a check
is made that the discriminants read from the stream are equal to those
of the actual parameter. Constraint_Error is raised if this check fails.
{
AI95-00195-01}
{unspecified [partial]} It
is unspecified at which point and in which order these checks are performed.
In particular, if Constraint_Error is raised due to the failure of one
of these checks, it is unspecified how many stream elements have been
read from the stream.
{
8652/0045}
{
AI95-00132-01}
{End_Error (raised by failure of run-time
check)} In the default implementation
of Read and Input for a type, End_Error is raised if the end of the stream
is reached before the reading of a value of the type is completed.
{
8652/0040}
{
AI95-00108-01}
{
AI95-00195-01}
{
AI95-00251-01}
{specifiable (of Read for a type)
[partial]} {specifiable
(of Write for a type) [partial]} {specifiable
(of Input for a type) [partial]} {specifiable
(of Output for a type) [partial]} {Read
clause} {Write
clause} {Input
clause} {Output
clause} The stream-oriented attributes
may be specified for any type via an
attribute_definition_clause.
The subprogram name given in such a clause shall not denote an abstract
subprogram. Furthermore, if a stream-oriented attribute is specified
for an interface type by an
attribute_definition_clause,
the subprogram name given in the clause shall statically denote a null
procedure.
Discussion:
{
AI95-00251-01}
Stream attributes (other than Input) are always null procedures for interface
types (they have no components). We need to allow explicit setting of
the Read and Write attributes in order that the class-wide attributes
like LI'Class'Input can be made available. (In that case, any descendant
of the interface type would require available attributes.) But we don't
allow any concrete implementation because these don't participate in
extensions (unless the interface is the parent type). If we didn't ban
concrete implementations, the order of declaration of a pair of interfaces
would become significant. For example, if Int1 and Int2 are interfaces
with concrete implementations of 'Read, then the following declarations
would have different implementations for 'Read:
type Con1 is new Int1 and Int2 with null record;
type Con2 is new Int2 and Int1 with null record;
This would violate
our design principle that the order of the specification of the interfaces
in a
derived_type_definition
doesn't matter.
Ramification:
The Input attribute cannot be specified for an interface. As it is
a function, a null procedure is impossible; a concrete function is not
possible anyway as any function returning an abstract type must be abstract.
And we don't allow specifying stream attributes to be abstract subprograms.
This has no impact, as the availability of Int'Class'Input (where Int
is a limited interface) depends on whether Int'Read (not Int'Input) is
specified. There is no reason to allow Int'Output to be specified, either,
but there is equally no reason to disallow it, so we don't have a special
rule for that.
Discussion: {
AI95-00195-01}
Limited types generally do not have default implementations of the stream-oriented
attributes. The rules defining when a stream-oriented attribute is available
(see below) determine when an attribute of a limited type is in fact
well defined and usable. The rules are designed to maximize the number
of cases in which the attributes are usable. For instance, when the language
provides a default implementation of an attribute for a limited type
based on a specified attribute for the parent type, we want to be able
to call that attribute.
{
AI95-00195-01}
A stream-oriented attribute for a subtype of a specific type
T
is
available at places where one of the following conditions is
true:
{available (stream attribute)}
T is nonlimited.
The
attribute_designator
is Read (resp. Write) and
T is a limited record extension, and
the attribute Read (resp. Write) is available for the parent type of
T and for the types of all of the extension components.
Reason: In this case, the language provides
a well-defined default implementation, which we want to be able to call.
T is a limited untagged derived type, and
the attribute was inherited for the type.
Reason: Attributes are only inherited
for untagged derived types, and surely we want to be able to call inherited
attributes.
The
attribute_designator
is Input (resp. Output), and
T is a limited type, and the attribute
Read (resp. Write) is available for
T.
Reason: The default implementation of
Input and Output are based on Read and Write; so if the implementation
of Read or Write is good, so is the matching implementation of Input
or Output.
Reason: We always want to allow calling
a specified attribute. But we don't want availability to break privacy.
Therefore, only attributes whose specification can be seen count. Yes,
we defined the visibility of an
attribute_definition_clause
(see
8.3).
{
AI95-00195-01}
A stream-oriented attribute for a subtype of a class-wide type
T'Class
is available at places where one of the following conditions is true:
T is nonlimited;
the corresponding attribute of T is available,
provided that if T has a partial view, the corresponding attribute
is available at the end of the visible part where T is declared.
Reason: The rules are stricter for class-wide
attributes because (for the default implementation) we must ensure that
any specific attribute that might ever be dispatched to is available.
Because we require specification of attributes for extensions of limited
parent types with available attributes, we can in fact know this. Otherwise,
we would not be able to use default class-wide attributes with limited
types, a significant limitation.
Discussion: Stream attributes always
exist. It is illegal to call them in some cases. Having the attributes
not be defined for some limited types would seem to be a cleaner solution,
but it would lead to contract model problems for limited private types.
T'Input is available for abstract types
so that T'Class'Input is available. But we certainly don't want
to allow calls that could create an object of an abstract type. Remember
that T'Class is never abstract, so the above legality rule doesn't
apply to it. We don't have to discuss whether the attribute is specified,
as it cannot be: any function returning the type would have to be abstract,
and we do not allow specifying an attribute with an abstract subprogram.
{
AI95-00195-01}
In the
parameter_and_result_profiles
for the stream-oriented attributes, the subtype of the Item parameter
is the base subtype of
T if
T is a scalar type, and the
first subtype otherwise. The same rule applies to the result of the Input
attribute.
{
AI95-00195-01}
For an
attribute_definition_clause
specifying one of these attributes, the subtype of the Item parameter
shall be the base subtype if scalar, and the first subtype otherwise.
The same rule applies to the result of the Input function.
Reason: This is to simplify implementation.
Ramification: The view of the type at
the point of the
attribute_definition_clause
determines whether the first subtype or base subtype is required. Thus,
for a scalar type with a partial view (which is never scalar), whether
the first subtype or the base subtype is required is determined by whether
the
attribute_definition_clause
occurs before or after the full definition of the scalar type.
{
AI95-00366-01}
{support external streaming}
{external streaming
(type supports)} [A type is said to
support
external streaming if Read and Write attributes are provided for
sending values of such a type between active partitions, with Write marshalling
the representation, and Read unmarshalling the representation.] A limited
type supports external streaming only if it has available Read and Write
attributes. A type with a part that is of an access type supports external
streaming only if that access type or the type of some part that includes
the access type component, has Read and Write attributes that have been
specified via an
attribute_definition_clause,
and that
attribute_definition_clause
is visible. [An anonymous access type does not support external streaming.
]All other types support external streaming.
Ramification: A limited type with a part
that is of an access type needs to satisfy both rules.
Erroneous Execution
{
AI95-00279-01}
{
AI95-00344-01}
{erroneous execution (cause) [partial]}
If the internal tag returned by Descendant_Tag to
T'Class'Input identifies a type that is not library-level and whose tag
has not been created, or does not exist in the partition at the time
of the call, execution is erroneous.
Ramification: The definition of Descendant_Tag
prevents such a tag from being provided to T'Class'Input if T is a library-level
type. However, this rule is needed for nested tagged types.
Implementation Requirements
{
8652/0040}
{
AI95-00108-01}
For every subtype
S of a language-defined nonlimited specific
type
T, the output generated by S'Output or S'Write shall be readable
by S'Input or S'Read, respectively. This rule applies across partitions
if the implementation conforms to the Distributed Systems Annex.
{
AI95-00195-01}
If Constraint_Error is raised during a call to Read because of failure
of one the above checks, the implementation must ensure that the discriminants
of the actual parameter of Read are not modified.
Implementation Permissions
{
AI95-00195-01}
The number of calls performed by the predefined implementation of the
stream-oriented attributes on the Read and Write operations of the stream
type is unspecified. An implementation may take advantage of this permission
to perform internal buffering. However, all the calls on the Read and
Write operations of the stream type needed to implement an explicit invocation
of a stream-oriented attribute must take place before this invocation
returns. An explicit invocation is one appearing explicitly in the program
text, possibly through a generic instantiation (see
12.3).
33 For a definite subtype S of a type T,
only T'Write and T'Read are needed to pass an arbitrary
value of the subtype through a stream. For an indefinite subtype S of
a type T, T'Output and T'Input will normally be
needed, since T'Write and T'Read do not pass bounds, discriminants,
or tags.
34 User-specified attributes of S'Class
are not inherited by other class-wide types descended from S.
Examples
Example of user-defined
Write attribute:
{
AI95-00441-01}
procedure My_Write(
Stream :
not null access Ada.Streams.Root_Stream_Type'Class;
Item : My_Integer'Base);
for My_Integer'Write
use My_Write;
Discussion:
Example of network input/output using input output attributes:
with Ada.Streams; use Ada.Streams;
generic
type Msg_Type(<>) is private;
package Network_IO is
-- Connect/Disconnect are used to establish the stream
procedure Connect(...);
procedure Disconnect(...);
-- Send/Receive transfer messages across the network
procedure Send(X : in Msg_Type);
function Receive return Msg_Type;
private
type Network_Stream is new Root_Stream_Type with ...
procedure Read(...); -- define Read/Write for Network_Stream
procedure Write(...);
end Network_IO;
with Ada.Streams; use Ada.Streams;
package body Network_IO is
Current_Stream : aliased Network_Stream;
. . .
procedure Connect(...) is ...;
procedure Disconnect(...) is ...;
procedure Send(X : in Msg_Type) is
begin
Msg_Type'Output(Current_Stream'Access, X);
end Send;
function Receive return Msg_Type is
begin
return Msg_Type'Input(Current_Stream'Access);
end Receive;
end Network_IO;
Inconsistencies With Ada 95
{
8652/0040}
{
AI95-00108-01}
{
inconsistencies with Ada 95}
Corrigendum:
Clarified how the default implementation for stream attributes is determined
(eliminating conflicting language). The new wording provides that attributes
for type extensions are created by composing the parent's attribute with
those for the extension components if any. If a program was written assuming
that the extension components were not included in the stream (as in
original Ada 95), it would fail to work in the language as corrected
by the Corrigendum.
{
AI95-00195-01}
Amendment Correction: Explicitly provided a permission that the
number of calls to the underlying stream Read and Write operations may
differ from the number determined by the canonical operations. If Ada
95 code somehow depended on the number of calls to Read or Write, it
could fail with an Ada 2005 implementation. Such code is likely to be
very rare; moreover, such code is really wrong, as the permission applies
to Ada 95 as well.
Extensions to Ada 95
{
AI95-00270-01}
{
extensions to Ada 95}
The Stream_Size attribute
is new. It allows specifying the number of bits that will be streamed
for a type. The Implementation Advice involving this also was changed;
this is not incompatible because Implementation Advice does not have
to be followed.
{
8652/0040}
{
AI95-00108-01}
{
AI95-00195-01}
{
AI95-00444-01}
Corrigendum: Limited types may have default constructed attributes
if all of the parent and (for extensions) extension components have available
attributes. Ada 2005 adds the notion of availability to patch up some
holes in the Corrigendum model.
Wording Changes from Ada 95
{
8652/0045}
{
AI95-00132-01}
Corrigendum: Clarified that End_Error is raised by the default
implementation of Read and Input if the end of the stream is reached.
(The result could have been abnormal without this clarification, thus
this is not an inconsistency, as the programmer could not have depended
on the previous behavior.)
{
AI95-00195-01}
Clarified that the default implementation of S'Input does normal initialization
on the object that it passes to S'Read.
{
AI95-00195-01}
Explicitly stated that what is read from a stream when a required check
fails is unspecified.
{
AI95-00251-01}
Defined availability and default implementations for types with progenitors.
{
AI95-00279-01}
Specified that Constraint_Error is raised if the internal tag retrieved
for S'Class'Input is for some type not covered by S'Class or is abstract.
We also explicitly state that the program is erroneous if the tag has
not been created or does not currently exist in the partition. (Ada 95
did not specify what happened in these cases; it's very unlikely to have
provided some useful result, so this is not considered an inconsistency.)
{
AI95-00344-01}
Added wording to support nested type extensions. S'Input and S'Output
always raise Tag_Error for such extensions, and such extensions were
not permitted in Ada 95, so this is neither an extension nor an incompatibility.
{
AI95-00366-01}
Defined
supports external streaming to put all of the rules about
“good” stream attributes in one place. This is used for distribution
and for defining pragma Pure.
{
AI95-00441-01}
Added the
not null qualifier to the first parameter of all of
the stream attributes, so that the semantics doesn't change between Ada
95 and Ada 2005. This change is compatible, because mode conformance
is required for subprograms specified as stream attributes, and
null_exclusions
are not considered for mode conformance.
{
AI95-00444-01}
Improved the wording to make it clear that we don't define the default
implementations of attributes that cannot be called (that is, aren't
“available”). Also clarified when inheritance takes place.
Sponsored by Ada-Europe