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Packages

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In Ada, packages are a fundamental way to organize and modularize code. They allow you to group related declarations and implementations together, making your code more manageable and reusable. This section will cover the basics of creating and using packages in Ada.

Key Concepts

  1. Package Specification: The interface of the package, which declares the types, variables, constants, and subprograms that are available to users of the package.
  2. Package Body: The implementation of the package, where the actual code for the subprograms and other entities declared in the specification is written.
  3. Visibility: The scope in which the entities declared in the package specification are accessible.
  4. Encapsulation: Hiding the implementation details from the user, exposing only what is necessary.

Package Structure

A package in Ada consists of two parts:

  1. Package Specification: This is the interface of the package.
  2. Package Body: This contains the implementation details.

Example

Let's create a simple package for mathematical operations.

Package Specification

-- math_operations.ads
package Math_Operations is
   function Add (X, Y : Integer) return Integer;
   function Subtract (X, Y : Integer) return Integer;
end Math_Operations;

Package Body

-- math_operations.adb
package body Math_Operations is
   function Add (X, Y : Integer) return Integer is
   begin
      return X + Y;
   end Add;

   function Subtract (X, Y : Integer) return Integer is
   begin
      return X - Y;
   end Subtract;
end Math_Operations;

Using the Package

To use the Math_Operations package in your Ada program, you need to with the package and then use it.

with Ada.Text_IO; use Ada.Text_IO;
with Math_Operations; use Math_Operations;

procedure Main is
   Result : Integer;
begin
   Result := Add(10, 5);
   Put_Line("10 + 5 = " & Integer'Image(Result));

   Result := Subtract(10, 5);
   Put_Line("10 - 5 = " & Integer'Image(Result));
end Main;

Practical Exercises

Exercise 1: Creating a Package

Task: Create a package named String_Operations that provides two functions: Concat to concatenate two strings and Length to return the length of a string.

Solution:

Package Specification

-- string_operations.ads
package String_Operations is
   function Concat (S1, S2 : String) return String;
   function Length (S : String) return Integer;
end String_Operations;

Package Body

-- string_operations.adb
package body String_Operations is
   function Concat (S1, S2 : String) return String is
   begin
      return S1 & S2;
   end Concat;

   function Length (S : String) return Integer is
   begin
      return S'Length;
   end Length;
end String_Operations;

Exercise 2: Using the Package

Task: Write a program that uses the String_Operations package to concatenate two strings and print the result along with its length.

Solution:

with Ada.Text_IO; use Ada.Text_IO;
with String_Operations; use String_Operations;

procedure Main is
   S1, S2, Result : String := "Hello, ";
   Length_Result : Integer;
begin
   S2 := "World!";
   Result := Concat(S1, S2);
   Length_Result := Length(Result);

   Put_Line("Concatenated String: " & Result);
   Put_Line("Length of Concatenated String: " & Integer'Image(Length_Result));
end Main;

Common Mistakes and Tips

  • Forgetting to with and use the package: Ensure you include the package in your program using the with and use clauses.
  • Mismatched package specification and body: Ensure that the declarations in the package specification match those in the package body.
  • Encapsulation: Use packages to encapsulate related functionality and hide implementation details from the user.

Conclusion

In this section, we learned about the structure and usage of packages in Ada. Packages help in organizing code, promoting reusability, and encapsulating implementation details. We also created a simple package and used it in a program. In the next section, we will explore generic units, which allow for the creation of flexible and reusable code components.

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