A.12.1 The Package Streams.Stream_IO

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The subprograms in the child package Streams.Stream_IO provide control over stream files. Access to a stream file is either sequential, via a call on Read or Write to transfer an array of stream elements, or positional (if supported by the implementation for the given file), by specifying a relative index for an element. Since a stream file can be converted to a Stream_Access value, calling stream-oriented attribute subprograms of different element types with the same Stream_Access value provides heterogeneous input-output. See 13.13 for a general discussion of streams.

Static Semantics

The elements of a stream file are stream elements. If positioning is supported for the specified external file, a current index and current size are maintained for the file as described in A.8. If positioning is not supported, a current index is not maintained, and the current size is implementation defined.

The library package Streams.Stream_IO has the following declaration:

with Ada.IO_Exceptions;
package Ada.Streams.Stream_IO is

    type Stream_Access is access all Root_Stream_Type'Class;

    type File_Type is limited private;

    type File_Mode is (In_File, Out_File, Append_File);

    type    Count          is range 0 .. implementation-defined; 
    subtype Positive_Count is Count range 1 .. Count'Last; 
       -- Index into file, in stream elements.

    procedure Create (File : in out File_Type; 
                      Mode : in File_Mode := Out_File; 
                      Name : in String    := ""; 
                      Form : in String    := "");

    procedure Open (File : in out File_Type;
                    Mode : in File_Mode;
                    Name : in String; 
                    Form : in String := "");

    procedure Close  (File : in out File_Type);
    procedure Delete (File : in out File_Type); 
    procedure Reset  (File : in out File_Type; Mode : in File_Mode); 
    procedure Reset  (File : in out File_Type);

    function Mode (File : in File_Type) return File_Mode; 
    function Name (File : in File_Type) return String; 
    function Form (File : in File_Type) return String;

    function Is_Open     (File : in File_Type) return Boolean; 
    function End_Of_File (File : in File_Type) return Boolean;

    function Stream (File : in File_Type) return Stream_Access; 
        -- Return stream access for use with T'Input and T'Output

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    -- Read array of stream elements from file
    procedure Read (File : in  File_Type;
                    Item : out Stream_Element_Array; 
                    Last : out Stream_Element_Offset; 
                    From : in  Positive_Count);

    procedure Read (File : in  File_Type;
                    Item : out Stream_Element_Array; 
                    Last : out Stream_Element_Offset);

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    -- Write array of stream elements into file
    procedure Write (File : in File_Type;
                     Item : in Stream_Element_Array; 
                     To   : in Positive_Count);

    procedure Write (File : in File_Type; 
                     Item : in Stream_Element_Array);

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    -- Operations on position within file

    procedure Set_Index(File : in File_Type; To : in Positive_Count);

    function Index(File : in File_Type) return Positive_Count; 
    function Size (File : in File_Type) return Count;

    procedure Set_Mode(File : in out File_Type; Mode : in File_Mode);

    procedure Flush(File : in File_Type);

    -- exceptions
    Status_Error : exception renames IO_Exceptions.Status_Error; 
    Mode_Error   : exception renames IO_Exceptions.Mode_Error; 
    Name_Error   : exception renames IO_Exceptions.Name_Error; 
    Use_Error    : exception renames IO_Exceptions.Use_Error; 
    Device_Error : exception renames IO_Exceptions.Device_Error; 
    End_Error    : exception renames IO_Exceptions.End_Error; 
    Data_Error   : exception renames IO_Exceptions.Data_Error;

private
    ... -- not specified by the language
end Ada.Streams.Stream_IO;

The subprograms Create, Open, Close, Delete, Reset, Mode, Name, Form, Is_Open, and End_of_File have the same effect as the corresponding subprograms in Sequential_IO (see A.8.2).

The Set_Mode procedure changes the mode of the file. If the new mode is Append_File, the file is positioned to its end; otherwise, the position in the file is unchanged.

The Flush procedure synchronizes the external file with the internal file (by flushing any internal buffers) without closing the file or changing the position. Mode_Error is propagated if the mode of the file is In_File.

The Stream function returns a Stream_Access result from a File_Type object, thus allowing the stream-oriented attributes Read, Write, Input, and Output to be used on the same file for multiple types. Stream propagates Status_Error if File is not open.

The procedures Read and Write are equivalent to the corresponding operations in the package Streams. Read propagates Mode_Error if the mode of File is not In_File. Write propagates Mode_Error if the mode of File is not Out_File or Append_File. The Read procedure with a Positive_Count parameter starts reading at the specified index. The Write procedure with a Positive_Count parameter starts writing at the specified index.

The Size function returns the current size of the file.

The Index function returns the current index.

The Set_Index procedure sets the current index to the specified value.

If positioning is supported for the external file, the current index is maintained as follows:

For Open and Create, if the Mode parameter is Append_File, the current index is set to the current size of the file plus one; otherwise, the current index is set to one.
For Reset, if the Mode parameter is Append_File, or no Mode parameter is given and the current mode is Append_File, the current index is set to the current size of the file plus one; otherwise, the current index is set to one.
For Set_Mode, if the new mode is Append_File, the current index is set to current size plus one; otherwise, the current index is unchanged.
For Read and Write without a Positive_Count parameter, the current index is incremented by the number of stream elements read or written.
For Read and Write with a Positive_Count parameter, the value of the current index is set to the value of the Positive_Count parameter plus the number of stream elements read or written.

If positioning is not supported for the given file, then a call of Index or Set_Index propagates Use_Error. Similarly, a call of Read or Write with a Positive_Count parameter propagates Use_Error.

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Erroneous Execution

If the File_Type object passed to the Stream function is later closed or finalized, and the stream-oriented attributes are subsequently called (explicitly or implicitly) on the Stream_Access value returned by Stream, execution is erroneous. This rule applies even if the File_Type object was opened again after it had been closed.

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