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Author:halw

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Date:2008-10-22T21:23:04.000000Z


git-svn-id: https://svn.eiffel.com/eiffel-org/trunk@94 abb3cda0-5349-4a8f-a601-0c33ac3a8c38
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halw
2008-10-23 14:33:54 +00:00
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12 changed files with 206 additions and 210 deletions
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documentation/current/method/eiffel-tutorial-et/et-other-mechanisms.wiki
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@@ -32,7 +32,7 @@ For the classes using it, <code>console</code>, although a function, looks very
The "Hello World" system at the beginning of this discussion (section [[4 Hello World|4]] ) used an output instruction of the form <code>io</code>. <code>put_string ( Some string )</code>. This is another example of the general scheme illustrated by <code>console</code>. Feature <code>io</code>, declared in <code>ANY</code> and hence usable by all classes, is a once function that returns an object of type <code>STANDARD_FILES</code> (another Kernel Library class) providing access to basic input and output features, one of which is procedure <code>put_string</code>. Because basic input and output must all work on the same files, <code>io</code> should clearly be a <code>once</code> function, shared by all classes that need these mechanisms.
==Constant and unique attributes==
==Constant attributes==
The attributes studied earlier were variable: each represents a field present in each instance of the class and changeable by its routines.
@@ -45,15 +45,6 @@ These will have the same value for every instance and hence do not need to occup
What comes after the <code>=</code> is a manifest constant: a self-denoting value of the appropriate type. Manifest constants are available for integers, reals (also used for doubles), booleans ( <code>True</code> and <code>False</code>), characters (in single quotes, as <code>'A'</code>, with special characters expressed using a percent sign as in <code>'%N'</code> for new line, <code>'%B'</code> for backspace and <code>'%U'</code> for null).
For integer constants, it is also possible to avoid specifying the values. A declaration of the form
<code>
a, b, c, ... n : INTEGER = unique
</code>
introduces <code>a</code>, <code>b</code>, <code>c</code>, ... <code>n</code> as constant integer attributes, whose value are assigned by the Eiffel compiler rather than explicitly by the programmer. The values are different for all <code>unique</code> attributes in a system; they are all positive, and, in a single declaration such as the above, guaranteed to be consecutive (so that you may use an invariant property of the form <code>code > a and code < n</code> to express that <code>code</code> should be one of the values). This mechanism replaces the "enumerated types" found in many languages, without suffering from the same problems. (Enumerated types have an ill-defined place in the type system; and it is not clear what operations are permitted.)
You may use Unique values in conjunction with the <code>inspect</code> multi-branch instruction studied in the next section. They are only appropriate for codes that can take on a fixed number of well-defined values -- not as a way to program operations with many variants, a need better addressed by the object-oriented technique studied earlier and relying on inheritance, polymorphism, redeclaration and dynamic binding.
Manifest constants are also available for strings, using double quotes as in
<code>
User_friendly_error_message: STRING is "Go get a life !"