Object-oriented Software in Ada 95

278 Input and output
Which when run would print the following results:
Mike 183 Male
Corinna 171 Female
Miranda 74 Female
18.4 Switching the default input and output streams
It is possible to switch the default input or output stream to another stream using the following procedures in the
package Ada.Text_Io. The effect is to change the source or sink from which input and output will come from
or go to when using the normal input and output procedures put and get without a File parameter.
Procedure
Set_Input ( File:in File_Type )
Set_OutPut( File:in File_Type )
Set_Error ( File:in File_Type )
Sets the default
Input file descriptor.
Output file descriptor.
Error file descriptor.
As the file descriptor is of type limited private, it may not be directly assigned. However, an access value of
the file descriptor can be saved. The following functions return an access value of the standard input and output
file descriptors:
Function
Standard_Input return File_Access;
Standard_Output return File_Access;
Standard_Error return File_Access;
Returns the access
Value of the input file descriptor.
Value of the output file descriptor.
Value of the error file descriptor.
18.4.1 Putting it all together
The program first saves an access value to the original default input stream. Then it switches the default input
stream to be the file file.txt. After reading and printing the contents of this file the default, input stream is
switched back to its original value and the contents of the stream are read and written out.
with Ada.Text_Io; use Ada.Text_Io;
procedure Main is
Fd : Ada.Text_Io.File_Type; --File descriptor
P_St_Fd : Ada.Text_Io.File_Access; --Access value of Standard
Ch : Character; --Current character
begin
P_St_Fd := Standard_Input; --Acess value of standard fd
Open( File=>Fd, Mode=>In_File, Name=>"file.txt" );
Set_Input( Fd );
while not End_Of_File loop --For each Line
while not End_Of_Line loop -- For each character
Get(Ch); Put(Ch); -- Read / Write character
end loop;
Skip_Line; New_Line; -- Next line / new line
end loop;
Close(Fd);
--Close file
Set_Input( P_St_Fd.all );
while not End_Of_File loop --For each Line
while not End_Of_Line loop --For each character
Get(Ch); Put(Ch); --Read / Write character
end loop;
Skip_Line; New_Line;
end loop;
end Main;
--Next line / new line
Note:
Notice how .all has been used to de-reference the access value when using the procedure
Set_Input.
© M A Smith - May not be reproduced without permission