Object-oriented Software in Ada 95

Procedures and functions 65
Mode Allowed as a Effect
parameter to:
in a function or a
procedure.
The formal parameter is initialized to the contents of the actual
parameter and may only be read from.
in out only a procedure The formal parameter is initialized to the contents of the actual
parameter and may be read from or written to. When the
procedure is exited, the new value of the formal parameter
replaces the old contents of the actual parameter.
out only a procedure The formal parameter is not initialized to the contents of the
actual parameter and may be read from or written to. When the
procedure is exited, the new value of the formal parameter
replaces the old contents of the actual parameter.
In Ada 83 an out formal parameter may not be read from.
Note:
The implementation of the above for simple objects is usually performed by copying the contents of the
object, whilst for large objects the compiler may implement this by using references to the actual
object.
5.5.1 Example of mode in out
A procedure swap which interchanges the contents of the actual parameters passed to it is as follows:
procedure Swap(First:in out Integer; Second:in out Integer) is
Temp : Integer;
begin
Temp := First;
First := Second; Second := Temp;
end Swap;
5.5.2 Putting it all together
The function swap may then be used in a program as follows:
with Ada.Text_Io, Ada.Integer_Text_Io, Swap;
use Ada.Text_Io, Ada.Integer_Text_Io;
procedure Main is
Books_Room_1 : Integer;
Books_Room_2 : Integer;
begin
Books_Room_1 := 10; Books_Room_2 := 20;
Put("Books in room 1 ="); Put(Books_Room_1); New_Line;
Put("Books in room 2 ="); Put(Books_Room_2); New_Line;
Put("Swap around"); New_Line;
Swap(Books_Room_1, Books_Room_2);
Put("Books in room 1 ="); Put(Books_Room_1); New_Line;
Put("Books in room 2 ="); Put(Books_Room_2); New_Line;
end Main;
which when run produces:
Books in room 1 = 10
Books in room 2 = 20
Swap around
Books in room 1 = 20
Books in room 2 = 10
© M A Smith - May not be reproduced without permission