Parameters to Generic Packages and Procedures

Parameters for Generic Packages and Procedures

The Problem

• Generic packages and generic procedures can have types as parameters, and


• Ada has a very rich type model


• Thus, need a notation to specify what kind of type is allowed as a generic parameter


• Question: Can a parameter to a generic have a default value?

Generic Parameters that are Objects

• A Generic parameter that is an object

    generic
       size : in Natural;
    

• This uses the term object to mean something that has a value
• It does NOT mean object in the way it's used in Object Oriented Programming
• Although OO objects also have values

Generic Parameters that are Types

• Possible generic parameters that are types

    generic
        type X is   limited private                 -- any type at all
        type X is   private                         -- any type that is not limited 
                   
        type X is   (<>)                            -- any discrete (integer or enumeration) type
        type X is   range <>                        -- any signed integer type
        type X is   mod <>                          -- any modular integer type
        type X is   digits <>                       -- any floating point type
        type X is   delta <>                        -- any fixed point type
        type X is   delta <> digits <>              -- any decimal type
                   
        type X is   access Y                        -- any access-to-Y type
        type X is   access all Y                    -- any "access all Y" type
        type X is   access constant Y               -- any "access constant Y" type
                   
        type X is   array (Y range <>) of Z         -- any UNCONSTRAINED array-of-Z type with a subtype of Y as its index subtype
        type X is   array (Y) of Z                  -- any CONSTRAINED array-of-Z type with a subtype of Y as its index subtype
                   
        type X is   new Y                           -- any type derived from Y
        type X is   new Y with private              -- any non-abstract tagged type derived from Y
                   
        type X is   abstract new Y with private     -- any tagged type (abstract or not) derived from Y
        type X is   tagged private                  -- any non-abstract non-limited tagged type
        type X is   tagged limited private          -- any non-abstract tagged type
        type X is   abstract tagged private         -- any tagged type that is not limited
        type X is   abstract tagged limited private -- any tagged type at all
    

Generic Parameters that are Procedures and Functions

• Parameters to generic packages (and procedures and functions) can also be:
• Procedures and functions, themselves


• Example:

        generic
            type ItemType is private;

            with function "="(Left, Right: ItemType) return Boolean;
        package SomePkg is
        ...
        end SomePkg;
        ...
        package body SomePkg is
           ...
           procedure x(s1, s2: ItemType) is
           begin
              if "="(s1, s2) then
              ...
              if s1 = s2 then

        end SomePkg;
        

• Client:

        type Student is record ...

        function eq(x, y: Student) return boolean is
        begin
            return x.ID = y.ID;
        end eq;

        package myPkg is new SomePkg(
            ItemType => Student, 
            "=" => eq
        );


        

Generic Parameters that are Packages

• Parameters to generic packages (and procedures and functions) can also be instantiations of other generic packages


• Example:

        generic
            type ItemType is private;

            package whatever is new SomePkg(<>);

        package SomePkg2 is
        

Default Values for Generic Parameters

• It's simple to declare a default value for a parameter for a generic


• Examples:

       generic 
          Object : in Natural := 0;

         type ItemType is private;

          with function "="(Left, Right: ItemType) return Boolean is <>  
       package MyPkg is
    



• The Box (ie <>) specifies that the default = for ItemType is used if this parameter is not specified