One of the main principles of modular programming, as well as object oriented programming, is encapsulation.
Encapsulation, briefly, is the concept that the implementer of a piece of software will distinguish between the code's public interface and its private implementation.
This is not only applicable to software libraries but wherever abstraction is used.
In Ada, the granularity of encapsulation is a bit different from most object-oriented languages, because privacy is generally specified at the package level.
Abstract data types¶
With this high-level granularity, it might not seem obvious how to hide the implementation details of a type. Here is how it can be done in Ada:
In the above example, we define a stack type in the public part (known as the visible part of the package spec in Ada), but the exact representation of that type is private.
Then, in the private part, we define the representation of that type. We can also declare other types that will be used as helpers for our main public type. This is useful since declaring helper types is common in Ada.
A few words about terminology:
- The Stack type as viewed from the public part is called the partial view of the type. This is what clients have access to.
- The Stack type as viewed from the private part or the body of the package is called the full view of the type. This is what implementers have access to.
From the point of view of the client (the with'ing unit), only the public (visible) part is important, and the private part could as well not exist. It makes it very easy to read linearly the part of the package that is important for you.
-- No need to read the private part to use the package package Stacks is type Stack is private; procedure Push (S : in out Stack; Val : Integer); procedure Pop (S : in out Stack; Val : out Integer); private ... end Stacks;
Here is how the
Stacks package would be used:
-- Example of use with Stacks; use Stacks; procedure Test_Stack is S : Stack; Res : Integer; begin Push (S, 5); Push (S, 7); Pop (S, Res); end Test_Stack;
Ada's limited type facility allows you to declare a type for which assignment and comparison operations are not automatically provided.
This is useful because, for example, for some data types the built-in assignment operation might be incorrect (for example when a deep copy is required).
Ada does allow you to overload the comparison operators
/= for limited
types (and to override the built-in declarations for non-limited types).
Ada also allows you to implement special semantics for assignment via
controlled types. However, in some cases
assignment is simply inappropriate; one example is the
File_Type from the
Ada.Text_IO package, which is declared as a limited type and thus
attempts to assign one file to another would be detected as illegal.