Keyword and Default Parameters

Keyword Parameters

Keyword Parameters

• A version of Ada.Integer_Text_IO.put has these parameters:
• Item - the integer to print
• Width - how many spaces to use to print the integer
• Base - the base to print (eg base 2 or base 16)


• Normal parameter passing is positional - matched by order


• Keyword parameters can be used - matched by parameter name!
• All of the following are legal, except the last:

   put(i);
   put(i, 1);       -- width 1
   put(i, 1, 2);    -- width 1, base 2

   put(Item => i);
   put(Item => i, Width => 1); 
   put(Item => i, Width => 1, Base => 2);

   put(Width => 1, Item => i); 
   put(Width => 1, Base => 2, Item => i);

   -- Mix positional and keyword.  Position must be first
   put(i, Width => 1); 
   put(i, Width => 1, Base => 2);
   put(i, 1, Base => 2);

   --  put(Width => 1, i); -- Error
   

Default Parameters

Default Parameters

• The declaration for Ada.Text_IO.put is similar to this:

   procedure Put(Item  : Num;
                 Width : Field := Default_Width;
                 Base  : Number_Base := Default_Base);
   

• The Width parameter has the default value Default_Width
• If put is called without a width, then the default value is used
• What is the default width?


• The Base parameter has the default value Default_Base
• What is the default base?
• So the following are all the same:

   put(i);            -- width 11, base 10
   put(i, 11);        -- width 11, base 10
   put(i, 11, 10);    -- width 11, base 10

   -- Check the values
   put(default_width);  -- 11
   put(default_base);   -- 10
   

OLD NOTES ON Keyword, Positional, and Default Parameters

Motivation

• Concern for programmer: easier to read and write
• Reliability: mistakes less likely

Keyword Parameters

• Ada.Integer_Text_IO.put is declared something like this:

          procedure put(Item: Integer; width: Natural) ...  

• Positional parameters are matched by order
• put(n, 5); -- n matched with parameter item, 5 with width


• Keyword parameters are matched by name
• put(item => n, width => 5);
• put(item => n, base => 2, width => 5);
• put(width => 5, item => n);
• put(item = > x, fore= > 5, aft= > 2, exp= > 0);


• Can mix positional and keyword: positional must be first
• put(n, width => 5);

Default Parameters

• Parameters can have default values


• Example: for new_line, the default is 1

        procedure new_line(Count: Positive := 1) ...  



• For integer_text_io, the default width is 11 (big enough for 2 billion and a sign)

        procedure put(Item: Integer; width: Natural := 11) ...