Section 2.2 - Declarations and Bodies

• The declaration, which contains information that will be visible to other program units. The declaration defines the interface for a program unit. Sometimes people refer to a declaration as a specification. They are somewhat analogous to the contents of C ``.h'' files.
• The body, which contains implementation details that need not be visible to other parts. They are somewhat analogous to the contents of C ``.c'' files.

These separate parts of program units are usually stored in separate files. This explicit distinction between declaration and body allows a program to be designed, written, and tested as a set of largely independent software components.

There are two special cases to help make programming easier:

1. Separate declarations are not required for subprograms (procedures and functions). If a subprogram has a body but no declaration, the body of a subprogram can serve as its own declaration. This makes writing the `hello, world' program in lesson 1 easier - technically, that simple program is a procedure body that automatically gives its own declaration.
2. For some packages, it's not possible to have implementation details. For example, a package declaration could be just a collection of constants (like pi and the square root of 2). In this case, the package must not have a body, since one isn't needed. This is relatively rare - most packages need both a declaration and a body.

Quiz:

Which part of a program unit contains the implementation details?

1. Declaration
2. Body

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