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13.13.2 Stream-Oriented Attributes
1/1
{
8652/0009}
The
operational attributes Write, Read, Output, and Input
attributes
convert values to a stream of elements and reconstruct values from a stream.
Static Semantics
2
For every subtype S of a specific type T,
the following attributes are defined.
3
- S'Write
-
S'Write denotes a procedure with
the following specification:
4
procedure S'Write(
Stream : access Ada.Streams.Root_Stream_Type'Class;
Item : in T)
5
- S'Write writes the value of Item to Stream.
6
- S'Read
-
S'Read denotes a procedure with
the following specification:
7
procedure S'Read(
Stream : access Ada.Streams.Root_Stream_Type'Class;
Item : out T)
8
- S'Read reads the value of Item from Stream.
8.1/1
{
8652/0040}
For untagged derived types, the Write and Read attributes of the parent type
are inherited as specified in 13.1; otherwise, the
default implementations of these attributes are used. The default implementations
of Write and Read attributes execute as follows:9/1
{
8652/0040}
For elementary types, the representation in terms of stream elements is implementation
defined. For composite types, the Write or Read attribute for each component
is called in
a canonical order
, which. The canonical order
of components 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 any ancestor
type of T has been directly specified and the attribute of any ancestor
type of the type of any of the extension components which are of a limited type
has not been specified, the attribute of T shall be directly specified.9.a
Implementation defined: The
representation used by the Read and Write attributes of elementary types
in terms of stream elements.
9.b
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.
9.b.1/1
{8652/0040}
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.
9.c
Ramification: 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.
10
For every subtype
S'Class of a class-wide type T'Class:
11
- S'Class'Write
-
S'Class'Write denotes a procedure
with the following specification:
12
procedure S'Class'Write(
Stream : access Ada.Streams.Root_Stream_Type'Class;
Item : in T'Class)
13
- Dispatches to the subprogram denoted by the Write
attribute of the specific type identified by the tag of Item.
14
- S'Class'Read
-
S'Class'Read denotes a procedure
with the following specification:
15
procedure S'Class'Read(
Stream : access Ada.Streams.Root_Stream_Type'Class;
Item : out T'Class)
16
- Dispatches to the subprogram denoted by the Read
attribute of the specific type identified by the tag of Item.
16.a
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
17
If a stream element is the same size as a storage
element, then the normal in-memory representation should be used by Read
and Write for scalar objects. Otherwise, Read and Write should use the
smallest number of stream elements needed to represent all values in
the base range of the scalar type.
Static Semantics
18
For every subtype S of a specific type T,
the following attributes are defined.
19
- S'Output
-
S'Output denotes a procedure
with the following specification:
20
procedure S'Output(
Stream : access Ada.Streams.Root_Stream_Type'Class;
Item : in T)
21
- S'Output writes the value of Item to Stream,
including any bounds or discriminants.
21.a
Ramification: Note that
the bounds are included even for an array type whose first subtype is
constrained.
22
- S'Input
-
S'Input denotes a function with
the following specification:
23
function S'Input(
Stream : access Ada.Streams.Root_Stream_Type'Class)
return T
24
- S'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.
25/1
{
8652/0040}
For untagged derived types, the Output and Input attributes of the parent
type are inherited as specified in 13.1; otherwise,
the default implementations of these attributes are used. The default implementations
of Output and Input attributes execute as follows:Unless overridden by
an attribute_definition_clause, these subprograms
execute as follows:26
- 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).
27
- 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), initializes it with S'Read, and returns
the value of the object.
28
For every subtype
S'Class of a class-wide type T'Class:
29
- S'Class'Output
-
S'Class'Output denotes a procedure
with the following specification:
30
procedure S'Class'Output(
Stream : access Ada.Streams.Root_Stream_Type'Class;
Item : in T'Class)
31
- First writes the external tag of Item to Stream
(by calling String'Output(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.
32
- S'Class'Input
-
S'Class'Input denotes a function
with the following specification:
33
function S'Class'Input(
Stream : access Ada.Streams.Root_Stream_Type'Class)
return T'Class
34
- First reads the external tag from Stream and determines
the corresponding internal tag (by calling Tags.Internal_Tag(String'Input(Stream))
-- 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.
35
{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).
35.1/1
{
8652/0045}
{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.36/1
{
8652/0040}
{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.
All nonlimited types have default implementations for these operations. An
attribute_reference
for one of these attributes is illegal if the type is limited, unless the attribute
has been specified by an
attribute_definition_clause
or [(for a type extension)] the attribute has been specified for an ancestor
type. 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.
36.a
Reason: This is to simplify
implementation.
36.a.1/1
Discussion: {8652/0040}
``Specified'' includes inherited attributes, and default implementations
are never inherited. So, for untagged limited types, the second part of the
attribute_reference rule has the same meaning
as the first part. However, tagged types never inherit attributes, so the second
rule is needed so that the default implementations for the attributes can be
called when those are constructed from a directly specified ancestor.
Implementation Requirements
36.1/1
{
8652/0040}
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. 37
31 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.
38
32 User-specified attributes
of S'Class are not inherited by other class-wide types descended from
S.
Examples
39
Example of user-defined
Write attribute:
40
procedure My_Write(
Stream : access Ada.Streams.Root_Stream_Type'Class; Item : My_Integer'Base);
for My_Integer'Write use My_Write;
40.a
Discussion:
Example of network input/output using input output attributes:
40.b
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(...);
40.c
-- 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;
40.d
with Ada.Streams; use Ada.Streams;
package body Network_IO is
Current_Stream : aliased Network_Stream;
. . .
procedure Connect(...) is ...;
procedure Disconnect(...) is ...;
40.e
procedure Send(X : in Msg_Type) is
begin
Msg_Type'Output(Current_Stream'Access, X);
end Send;
40.f
function Receive return Msg_Type is
begin
return Msg_Type'Input(Current_Stream'Access);
end Receive;
end Network_IO;
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