ch06_02.htm

编程珍珠,里面很多好用的代码,大家可以参考学习呵呵,

it things like this:
<blockquote>
<pre class="programlisting">@newlist = upcase(@list1, @list2);
@newlist = upcase( split /:/, $var );</pre>
</blockquote>

Do not, however, be tempted to do this:
<blockquote>
<pre class="programlisting">(@a, @b) = upcase(@list1, @list2);   # WRONG</pre>
</blockquote>

Why not?  Because, like the flat incoming parameter list in <tt class="literal">@_</tt>, the
return list is also flat.  So this stores everything in <tt class="literal">@a</tt> and
empties out <tt class="literal">@b</tt> by storing the null list there.  See the later section <a href="ch06_03.htm#ch06-sect-pr">Section 6.3, "Passing References"</a> for alternatives.</p>






<h3 class="sect2">6.2.2. Error Indications</h3>

<p>
<a name="INDEX-1825"></a><a name="INDEX-1826"></a>
If you want your function to return in such a way that the caller will
realize there's been an error, the most natural way to do this in Perl
is to use a bare <tt class="literal">return</tt> statement without an argument.  That way
when the function is used in scalar context, the caller gets <tt class="literal">undef</tt>,
and when used in list context, the caller gets a null list.</p>

<p>
<a name="INDEX-1827"></a>
Under extraordinary circumstances, you might choose to raise an
exception to indicate an error.  Use this measure sparingly, though;
otherwise, your whole program will be littered with exception
handlers.  For example, failing to open a file in a generic
file-opening function is hardly an exceptional event.  However,
ignoring that failure might well be.  The <tt class="literal">wantarray</tt> built-in
returns <tt class="literal">undef</tt> if your function was called in void context, so you
can tell if you're being ignored:
<blockquote>
<pre class="programlisting">if ($something_went_awry) {
    return if defined wantarray;  # good, not void context.
    die "Pay attention to my error, you danglesocket!!!\n";
}</pre>
</blockquote>
</p>






<h3 class="sect2">6.2.3. Scoping Issues</h3>

<p>
<a name="INDEX-1828"></a><a name="INDEX-1829"></a><a name="INDEX-1830"></a>
Subroutines may be called recursively because each call gets its own
argument array, even when the routine calls itself.  If a subroutine
is called using the <tt class="literal">&amp;</tt> form, the argument list is optional.  If the
<tt class="literal">&amp;</tt> is used but the argument list is omitted, something special
happens: the <tt class="literal">@_</tt> array of the calling routine is supplied
implicitly.  This is an efficiency mechanism that new users may wish
to avoid.
<blockquote>
<pre class="programlisting">&amp;foo(1,2,3);    # pass three arguments
foo(1,2,3);     # the same

foo();          # pass a null list
&amp;foo();         # the same

&amp;foo;           # foo() gets current args, like foo(@_), but faster!
foo;            # like foo() if sub foo predeclared, else bareword "foo"</pre>
</blockquote>

Not only does the <tt class="literal">&amp;</tt> form make the argument list
optional, but it
also disables any prototype checking on the arguments you do provide.
This is partly for historical reasons and partly to provide a
convenient way to cheat if you know what you're doing.  See the section
<a href="ch06_04.htm#ch06-sect-proto">Section 6.4, "Prototypes"</a> later in this chapter.</p>

<p>
<a name="INDEX-1831"></a>
Variables you access from inside a function that haven't been declared
private to that function are not necessarily global variables; they
still follow the normal block-scoping rules of Perl.  As explained in <a href="ch02_05.htm#ch02-sect-names">Section 6.5, "Names"</a> in <a href="ch02_01.htm">Chapter 2, "Bits and Pieces"</a>, this means they
look first in the surrounding lexical scope (or scopes) for resolution,
then on to the single package scope.  From the viewpoint of a
subroutine, then, any <tt class="literal">my</tt> variables from an enclosing lexical scope
are still perfectly visible.</p>

<p>For example, the <tt class="literal">bumpx</tt> function below has access to the file-scoped
<tt class="literal">$x</tt> lexical variable because the scope where the <tt class="literal">my</tt> was
declared--the file itself--hasn't been closed off before the
subroutine is defined:
<blockquote>
<pre class="programlisting"># top of file
my $x = 10;         # declare and initialize variable
sub bumpx { $x++ }  # function can see outer lexical variable</pre>
</blockquote>
<a name="INDEX-1832"></a>
</p>

<p>
<a name="INDEX-1833"></a>
C and C++ programmers would probably think of <tt class="literal">$x</tt> as a "file
static" variable.  It's private as far as functions in other files
are concerned, but global from the perspective of functions declared
after the <tt class="literal">my</tt>.  C programmers who come to Perl looking
for what
they would call "static variables" for files or functions find no
such keyword in Perl.  Perl programmers generally avoid the word
"static", because static systems are dead and boring, and because
the word is so muddled in historical usage.</p>

<p>Although Perl doesn't include the word "static" in its lexicon,
Perl programmers have no problem creating variables that are private to a
function and persist across function calls.  There's just no special
word for these.  Perl's richer scoping primitives combine with
automatic memory management in ways that someone looking for a
"static" keyword might never think of trying.</p>

<p>
<a name="INDEX-1834"></a>
Lexical variables don't get automatically garbage collected just
because their scope has exited; they wait to get recycled until they're no
longer <em class="emphasis">used</em>, which is much more important.  To create private
variables that aren't automatically reset across function calls,
enclose the whole function in an extra block and put both the <tt class="literal">my</tt>
declaration and the function definition within that block.  You can
even put more than one function there for shared access to an
otherwise private variable:
<blockquote>
<pre class="programlisting">{
    my $counter = 0;
    sub next_counter { return ++$counter }
    sub prev_counter { return --$counter }
}</pre>
</blockquote>

As always, access to the lexical variable is limited to code within
the same lexical scope.  The names of the two functions, on the other hand,
are globally accessible (within the package), and, since they were
defined inside <tt class="literal">$counter</tt>'s scope, they can still access that
variable even though no one else can.</p>

<p>
<a name="INDEX-1835"></a><a name="INDEX-1836"></a><a name="INDEX-1837"></a><a name="INDEX-1838"></a>
If this function is loaded via <tt class="literal">require</tt> or <tt class="literal">use</tt>, then this is
probably just fine.  If it's all in the main program, you'll need to
make sure any run-time assignment to <tt class="literal">my</tt> is executed early enough, either
by putting the whole block before your main program, or alternatively, by
placing a <tt class="literal">BEGIN</tt> or <tt class="literal">INIT</tt> block around it to make sure it gets
executed before your program starts:
<blockquote>
<pre class="programlisting">BEGIN {
    my @scale = ('A' .. 'G');
    my $note  = -1;
    sub next_pitch { return $scale[ ($note += 1) %= @scale ] };
}</pre>
</blockquote>

The <tt class="literal">BEGIN</tt> doesn't affect the subroutine definition, nor does it affect
the persistence of any lexicals used by the subroutine.  It's just
there to ensure the variables get initialized before the subroutine
is ever called.  For more on declaring private and global variables,
see <tt class="literal">my</tt> and <tt class="literal">our</tt> respectively in <a href="ch29_01.htm">Chapter 29, "Functions"</a>.
The <tt class="literal">BEGIN</tt> and <tt class="literal">INIT</tt> constructs are explained in <a href="ch18_01.htm">Chapter 18, "Compiling"</a>.</p>



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