Derived types and sub types in Ada

Question

What are the differences?

Answer 1

First of all, terminology: it's "Ada", not "ADA" -- it's named after "Ada Lovelace"; it is not an acronym.

A subtype is compatible with its base type, so you can mix operands of the base type with operands of the base type. For example:

subtype Week_Days is Integer range 1..7;

Since this is a subtype, you can (for example) add 1 to a weekday to get the next weekday.

A derived type is a completely separate type that has the same characteristics as its base type. You cannot mix operands of a derived type with operands of the base type. If, for example, you used:

type Week_Day is new Integer range 1..7;

Then you would not be able to add an integer to a weekday to get another weekday. To do manipulations on a derived type, you'd normally define those manipulations yourself (e.g., create a package). At the same time, a derived type does "inherit" all the operations of its base type (even some that may not make sense) so you do still get addition.

Answer 2

From Wikibooks:

Subtypes of a given type will be compatible with each other.

A derived type is a new, full-blown type created from an existing one. Like any other type, it is incompatible with its parent; however, it inherits the primitive operations defined for the parent type.