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13.13.1 The Package Streams

Static Semantics

The abstract type Root_Stream_Type is the root type of the class of stream types. The types in this class represent different kinds of streams. A new stream type is defined by extending the root type (or some other stream type), overriding the Read and Write operations, and optionally defining additional primitive subprograms, according to the requirements of the particular kind of stream. The predefined stream-oriented attributes like T'Read and T'Write make dispatching calls on the Read and Write procedures of the Root_Stream_Type. (User-defined T'Read and T'Write attributes can also make such calls, or can call the Read and Write attributes of other types.) 
package Ada.Streams is
    pragma Pure(Streams);
{AI95-00161-01}     type Root_Stream_Type is abstract tagged limited private;
    pragma Preelaborable_Initialization(Root_Stream_Type);
{8652/0044} {AI95-00181-01}     type Stream_Element is mod implementation-defined;
    type Stream_Element_Offset is range implementation-defined;
    subtype Stream_Element_Count is
        Stream_Element_Offset range 0..Stream_Element_Offset'Last;
    type Stream_Element_Array is
        array(Stream_Element_Offset range <>) of aliased Stream_Element;
    procedure Read(
      Stream : in out Root_Stream_Type;
      Item   : out Stream_Element_Array;
      Last   : out Stream_Element_Offset) is abstract;
    procedure Write(
      Stream : in out Root_Stream_Type;
      Item   : in Stream_Element_Array) is abstract;
   ... -- not specified by the language
end Ada.Streams;
{AI95-00227-01} The Read operation transfers Item'Length stream elements from the specified stream to fill the array Item. Elements are transferred until Item'Length elements have been transferred, or until the end of the stream is reached. If any elements are transferred, the The index of the last stream element transferred is returned in Last. Otherwise, Item'First - 1 is returned in Last. Last is less than Item'Last only if the end of the stream is reached.
The Write operation appends Item to the specified stream.
Discussion: {AI95-00114-01} The index subtype of Stream_Element_Array is Stream_Element_Offset because we wish to allow maximum flexibility. Most Stream_Element_Arrays will probably have a lower bound of 0 or 1, but other lower bounds, including negative ones, make sense in some situations.
{AI95-00114-01} {AI05-0005-1} Note that there are some language-defined subprograms that fill part of a Stream_Element_Array, and return the index of the last element filled as a Stream_Element_Offset. The Read procedures declared here, Streams.Stream_IO (see A.12.1), and System.RPC (see E.5) behave in this manner. These will raise Constraint_Error if the resulting Last value is not in Stream_Element_Offset. This implies that the Stream_Element_Array passed to these subprograms should not have a lower bound of Stream_Element_Offset'First, because then a read of 0 elements would always raise Constraint_Error. A better choice of lower bound is 0 or 1.

Implementation Permissions

  {8652/0044} {AI95-00181-01} If Stream_Element'Size is not a multiple of System.Storage_Unit, then the components of Stream_Element_Array need not be aliased.
Ramification: {AI95-00114-01} If the Stream_Element'Size is less than the size of System.Storage_Unit, then components of Stream_Element_Array need not be aliased. This is necessary as the components of type Stream_Element size might not be addressable on the target architecture architechture.
39  See A.12.1, “The Package Streams.Stream_IO” for an example of extending type Root_Stream_Type.
40  {AI95-00227-01} If the end of stream has been reached, and Item'First is Stream_Element_Offset'First, Read will raise Constraint_Error.
Ramification: Thus, Stream_Element_Arrays should start at 0 or 1, not Stream_Element_Offset'First.

Extensions to Ada 95

{AI95-00161-01} Amendment Correction: Added pragma Preelaborable_Initialization to type Root_Stream_Type. 

Wording Changes from Ada 95

{8652/0044} {AI95-00181-01} Corrigendum: Stream elements are aliased presuming that makes sense.
{AI95-00227-01} Fixed the wording for Read to properly define the result in Last when no stream elements are transfered. 

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