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Constrained genericity: Difference between revisions

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Constrained genericity (view source)

Revision as of 19:35, 11 September 2010

217 bytes removed ,  13 years ago
→‎{{header|Ada}}: I think this solves the issue
(Removed assertion that this language features applies only to statically typed languages. (see J implementation for counter example.))
(→‎{{header|Ada}}: I think this solves the issue)
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=={{header|Ada}}==
 
{{incorrect|Ada|This doesn't create a generic type FoodBox, but a non-generic type in which all objects derived from Food can be stored (the example code demonstrates it: Instead of a foodbox only for bananas and another only for tomatoes, one can put bananas and tomatoes into the same food box).}}
Ada allows various constraints to be specified in parameters of a generics. A formal type constrained to be derived from certain base is one of them:
<lang ada>with Ada.Containers.Indefinite_Vectors;
 
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procedure Eat (Object : in out Food) is abstract;
 
end Nutrition;</lang>▼
package Food_Boxes is▼
 
with Ada.Containers;
with Nutrition;
 
generic
type New_Food is new Nutrition.Food;
▲ package Food_Boxes is
 
package Food_Vectors is
new Ada.Containers.Indefinite_Vectors
( Index_Type => Positive,
Element_Type => Food'ClassNew_Food
);
 
subtype Food_Box is Food_BoxesFood_Vectors.Vector;
▲end Nutrition;</lang>
 
The package Nutrition defines an interface of an eatable object, that is, the procedure Eat. Then a container package is instantiated with the elements to be of the class Food. I.e. the elements can be only the members of the class Food. Example of use:▼
end Food_Boxes;
</lang>
▲The package Nutrition defines an interface of an eatable object, that is, the procedure Eat. Then a generic container package is instantiateddefined with the elements to be of thesome classtype Food. I.e. the elements can be only the members of thederived classfrom Food. Example of use:
<lang ada>type Banana is new Food with null record;
overriding procedure Eat (Object : in out Banana) is null;
package Banana_Box is new Food_Boxes (Banana);
 
type Tomato is new Food with null record;
overriding procedure Eat (Object : in out Tomato) is null;</lang>
package Tomato_Box is new Tomato_Boxes (Tomato);
-- We have declared Banana and Tomato as a Food.
<lang ada> Lunch_Box : Food_Box;
</lang>
begin
The Tomato_Box can only contain tomatoes; the Banana_Box can only contain bananas. You can only create boxes of eatable objects.
Lunch_Box.Append (Banana'(null record));
Lunch_Box.Append (Banana'(null record));
Lunch_Box.Append (Tomato'(null record));</lang>
The lunch box contains two banana and one tomato.
 
=={{header|C sharp|C#}}==
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