Constrained genericity: Difference between revisions

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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:

Ada allows various constraints to be specified in parameters of 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;

⚫

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'Class~~

Element_Type => New_Food

);

subtype Food_Box is ~~Food_Boxes~~.Vector;

subtype Food_Box is Food_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 defined with the elements to be of some type derived from 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>~~

overriding procedure Eat (Object : in out Tomato) is null;

package Tomato_Box is new Tomato_Boxes (Tomato);

We have declared Banana and Tomato as a Food.

-- 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 (Tomato'(null record));</lang>

The lunch box contains two banana and one tomato.

=={{header|C sharp|C#}}==