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   9 <title>section 2.4: Declarations</title>
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  15 <H2>section 2.4: Declarations</H2>
  16
  17 page 40
  18 <p>You may wonder <em>why</em> variables must be declared before use.
  19 There are two reasons:
  20 </p><OL><li>It makes
  21 things somewhat
  22 easier on the compiler;
  23 it knows right away what kind of storage to allocate
  24 and what code to emit to store and manipulate
  25 each variable;
  26 it doesn't have to try to intuit the programmer's intentions.
  27 <li>It forces a bit of useful discipline on the programmer:
  28 you cannot introduce variables willy-nilly;
  29 you must think about them enough to pick appropriate types for them.
  30 (The compiler's error messages to you,
  31 telling you that you apparently forgot to declare a variable,
  32 are as often helpful as they are a nuisance:
  33 they're helpful when they tell you that you misspelled a variable,
  34 or forgot to think about exactly how you were going to use it.)
  35 </OL><p>Although there are a few places where
  36 ``certain declarations can be made implicitly by context'',
  37 making use of these removes the advantages of reason
  38 2
  39 above,
  40 so I recommend always declaring everything explicitly.
  41 </p><p>Most of the time,
  42 I recommend writing one declaration per line
  43 (as in the ``latter form'' on page 40).
  44 For the most part, the compiler doesn't care what order declarations are in.
  45 You can order the declarations alphabetically,
  46 or in the order that they're used,
  47 or to put related declarations next to each other.
  48 Collecting all variables of the same type together on one line
  49 essentially orders declarations by type,
  50 which isn't a very useful order
  51 (it's only slightly more useful than random order).
  52 </p><p>If you'd rather not remember the rules for default initialization
  53
  54 (namely that ``external or static variables are initialized to
  55 zero by default'' and ``automatic variables for which there is no
  56 initializer have... garbage values''),
  57 you can get in the habit of initializing everything.
  58 It never hurts to explicitly initialize something
  59 when it would have been implicitly initialized anyway,
  60 but forgetting to initialize something that needs it can be the
  61 source of frustrating bugs.
  62 </p><p>Don't worry about the distinction between ``external or static variables'';
  63 we haven't seen it yet.
  64 </p><p>One mild surprise is that <TT>const</TT> variables are <em>not</em>
  65 ``constant expressions'' as defined on page 38.
  66 You can't say something like
  67 <pre>   const int maxline = 1000;
  68         char line[maxline+1];           /* WRONG */
  69 </pre></p><hr>
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  78 This page by <a href="http://www.eskimo.com/~scs/">Steve Summit</a>
  79 // <a href="copyright.html">Copyright</a> 1995, 1996
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