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13.13.2 Stream-Oriented Attributes

The Write, Read, Output, and Input attributes convert values to a stream of elements and reconstruct values from a stream.

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:

   
           procedure S'Write(
              Stream : 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:

   
           procedure S'Read(
              Stream : access Ada.Streams.Root_Stream_Type'Class;
              Item : out T)

S'Read reads the value of Item from Stream.

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. 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 every subtype S'Class of a class-wide type T'Class:

S'Class'Write

S'Class'Write denotes a procedure with the following specification:

    
           procedure S'Class'Write(
              Stream : 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:

    
           procedure S'Class'Read(
              Stream : 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.

Implementation Advice

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

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:

    
           procedure S'Output(
              Stream : access Ada.Streams.Root_Stream_Type'Class;
              Item : in T)

S'Output writes the value of Item to Stream, including any bounds or discriminants.

S'Input

S'Input denotes a function with the following specification:

    
           function S'Input(
              Stream : access Ada.Streams.Root_Stream_Type'Class)
              return T

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.

Unless overridden by an attribute_definition_clause, these subprograms 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).

• 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.

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:

    
           procedure S'Class'Output(
              Stream : access Ada.Streams.Root_Stream_Type'Class;
              Item   : in T'Class)

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.

S'Class'Input

S'Class'Input denotes a function with the following specification:

    
           function S'Class'Input(
              Stream : access Ada.Streams.Root_Stream_Type'Class)
              return T'Class

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.

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 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).

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. 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.

NOTES

(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.

(32) User-specified attributes of S'Class are not inherited by other class-wide types descended from S.

Examples

Example of user-defined Write attribute:

    
       procedure My_Write(
         Stream : access Ada.Streams.Root_Stream_Type'Class; Item : My_Integer'Base);
       for My_Integer'Write use My_Write;

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