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Packages

Packages. Presented by Quentin Ochem. university.adacore.com. The Ada Package. A package is the base of software architecture in Ada It ’ s a semantic entity checked by the compiler It separates clearly a specification and an implementation. General Structure of a Package.

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Packages

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  1. Packages Presented by Quentin Ochem university.adacore.com

  2. The Ada Package • A package is the base of software architecture in Ada • It’s a semantic entity checked by the compiler • It separates clearly a specification and an implementation

  3. General Structure of a Package • Entities should be put in the body except if they have to be exported • The body is easier to change than the specification

  4. Example

  5. Accessing components of a package • Only entities declared in the public part are visible • Entities are referenced through the dot notation

  6. Child units • A child unit is an extension of a package • Can be used to organize the namespace or break big packages into pieces • Child units have visibility over parents • Generally speaking, it’s a good habit to break functionality into packages as much as possible

  7. Full dependencies (“with clause”) • “With clause” defines a dependency between two packages • Gives access to all the public declarations • Can be applied to the spec or the body • A dependency is normally done to a specification • “Specification with” applies to the body • “Specification with” applies to children • There is no other transitivity

  8. About the usage of the "with" keyword • With is a highly overloaded Ada reserved word • It can be used to declare a dependency between two units • It can be used to introduce an aspect • It can be used to extend a record • … and others situations as well

  9. Dependency shortcut (“use clause”) • Prefix may be overkill • The “use clause” allows omitting it • Can introduce ambiguities • Can be placed in any scope

  10. A Note about Operator Symbol Visibility • With only a “with” clause, operators on types will not be visible! • Use type will specifically allow visibility to operators of a type • Prefix notation is possible too

  11. A Package is a High Level Semantic Entity • The compiler is responsible for checking structural and semantic consistency

  12. (1/10)

  13. (1/10) compilation error Read is not implemented in the body

  14. (2/10)

  15. (2/10) compilation error a body can't be written in a package spec

  16. (3/10)

  17. (3/10) compilation error "with P1;" is needed on P2 for the declaration to work

  18. (4/10)

  19. (4/10) compilation error T is declared in the body of P1, not reachable from the outside

  20. (5/10)

  21. (5/10) compilation error there is a circularity between P1 and P2

  22. (6/10)

  23. (6/10) there is no circularityP1 depends only the spec of P2 the body of P2 depends on the spec of P1

  24. (7/10)

  25. (7/10) compilation error operators are not reachable for the lack of use clause on "Types" package

  26. (8/10)

  27. (8/10) no errors a child package has use-visibility over its parent

  28. (9/10)

  29. (9/10) a child always depends on its parent this create a circularity here

  30. (10/10)

  31. (10/10) no error the body of P1 depends on the specification of P1.Child

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