Tie::Hash, Tie::StdHash, Tie::ExtraHash - base class definitions for tied hashes

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See <a href="/5.30.3/perltie">perltie</a> for a list of the functions required in order to tie a hash to a package. The basic <b>Tie::Hash</b> package provides a <code>new</code> method, as well as methods <code>TIEHASH</code>, <code>EXISTS</code> and <code>CLEAR</code>. The <b>Tie::StdHash</b> and <b>Tie::ExtraHash</b> packages provide most methods for hashes described in <a href="/5.30.3/perltie">perltie</a> (the exceptions are <code>UNTIE</code> and <code>DESTROY</code>). They cause tied hashes to behave exactly like standard hashes, and allow for selective overwriting of methods. <b>Tie::Hash</b> grandfathers the <code>new</code> method: it is used if <code>TIEHASH</code> is not defined in the case a class forgets to include a <code>TIEHASH</code> method.</p> <p>For developers wishing to write their own tied hashes, the required methods are briefly defined below. See the <a href="/5.30.3/perltie">perltie</a> section for more detailed descriptive, as well as example code:</p> <dl> <dt id="TIEHASH-classname,-LIST"><a class="permalink" href="#TIEHASH-classname,-LIST">#</a><a id="TIEHASH"></a><a id="TIEHASH-classname-LIST"></a>TIEHASH classname, LIST</dt> <dd> <p>The method invoked by the command <code>tie %hash, classname</code>. Associates a new hash instance with the specified class. <code>LIST</code> would represent additional arguments (along the lines of <a href="/5.30.3/AnyDBM_File">AnyDBM_File</a> and compatriots) needed to complete the association.</p> </dd> <dt id="STORE-this,-key,-value"><a class="permalink" href="#STORE-this,-key,-value">#</a><a id="STORE"></a><a id="STORE-this-key-value"></a>STORE this, key, value</dt> <dd> <p>Store datum <i>value</i> into <i>key</i> for the tied hash <i>this</i>.</p> </dd> <dt id="FETCH-this,-key"><a class="permalink" href="#FETCH-this,-key">#</a><a id="FETCH"></a><a id="FETCH-this-key"></a>FETCH this, key</dt> <dd> <p>Retrieve the datum in <i>key</i> for the tied hash <i>this</i>.</p> </dd> <dt id="FIRSTKEY-this"><a class="permalink" href="#FIRSTKEY-this">#</a><a id="FIRSTKEY"></a>FIRSTKEY this</dt> <dd> <p>Return the first key in the hash.</p> </dd> <dt id="NEXTKEY-this,-lastkey"><a class="permalink" href="#NEXTKEY-this,-lastkey">#</a><a id="NEXTKEY"></a><a id="NEXTKEY-this-lastkey"></a>NEXTKEY this, lastkey</dt> <dd> <p>Return the next key in the hash.</p> </dd> <dt id="EXISTS-this,-key"><a class="permalink" href="#EXISTS-this,-key">#</a><a id="EXISTS"></a><a id="EXISTS-this-key"></a>EXISTS this, key</dt> <dd> <p>Verify that <i>key</i> exists with the tied hash <i>this</i>.</p> <p>The <b>Tie::Hash</b> implementation is a stub that simply croaks.</p> </dd> <dt id="DELETE-this,-key"><a class="permalink" href="#DELETE-this,-key">#</a><a id="DELETE"></a><a id="DELETE-this-key"></a>DELETE this, key</dt> <dd> <p>Delete the key <i>key</i> from the tied hash <i>this</i>.</p> </dd> <dt id="CLEAR-this"><a class="permalink" href="#CLEAR-this">#</a><a id="CLEAR"></a>CLEAR this</dt> <dd> <p>Clear all values from the tied hash <i>this</i>.</p> </dd> <dt id="SCALAR-this"><a class="permalink" href="#SCALAR-this">#</a><a id="SCALAR"></a>SCALAR this</dt> <dd> <p>Returns what evaluating the hash in scalar context yields.</p> <p><b>Tie::Hash</b> does not implement this method (but <b>Tie::StdHash</b> and <b>Tie::ExtraHash</b> do).</p> </dd> </dl> <h1 id="Inheriting-from-Tie::StdHash"><a class="permalink" href="#Inheriting-from-Tie::StdHash">#</a><a id="Inheriting"></a>Inheriting from <b>Tie::StdHash</b></h1> <p>The accessor methods assume that the actual storage for the data in the tied hash is in the hash referenced by <code>tied(%tiedhash)</code>. Thus overwritten <code>TIEHASH</code> method should return a hash reference, and the remaining methods should operate on the hash referenced by the first argument:</p> <pre><code>package ReportHash; our @ISA = 'Tie::StdHash'; sub TIEHASH { my $storage = bless {}, shift; warn "New ReportHash created, stored in $storage.\n"; $storage } sub STORE { warn "Storing data with key $_[1] at $_[0].\n"; $_[0]{$_[1]} = $_[2] }</code></pre> <h1 id="Inheriting-from-Tie::ExtraHash"><a class="permalink" href="#Inheriting-from-Tie::ExtraHash">#</a><a id="Inheriting1"></a>Inheriting from <b>Tie::ExtraHash</b></h1> <p>The accessor methods assume that the actual storage for the data in the tied hash is in the hash referenced by <code>(tied(%tiedhash))->[0]</code>. Thus overwritten <code>TIEHASH</code> method should return an array reference with the first element being a hash reference, and the remaining methods should operate on the hash <code>%{ $_[0]->[0] }</code>:</p> <pre><code>package ReportHash; our @ISA = 'Tie::ExtraHash'; sub TIEHASH { my $class = shift; my $storage = bless [{}, @_], $class; warn "New ReportHash created, stored in $storage.\n"; $storage; } sub STORE { warn "Storing data with key $_[1] at $_[0].\n"; $_[0][0]{$_[1]} = $_[2] }</code></pre> <p>The default <code>TIEHASH</code> method stores "extra" arguments to tie() starting from offset 1 in the array referenced by <code>tied(%tiedhash)</code>; this is the same storage algorithm as in TIEHASH subroutine above. Hence, a typical package inheriting from <b>Tie::ExtraHash</b> does not need to overwrite this method.</p> <h1 id="SCALAR,-UNTIE-and-DESTROY"><a class="permalink" href="#SCALAR,-UNTIE-and-DESTROY">#</a><a id="SCALAR1"></a><a id="SCALAR-UNTIE-and-DESTROY"></a><code>SCALAR</code>, <code>UNTIE</code> and <code>DESTROY</code></h1> <p>The methods <code>UNTIE</code> and <code>DESTROY</code> are not defined in <b>Tie::Hash</b>, <b>Tie::StdHash</b>, or <b>Tie::ExtraHash</b>. Tied hashes do not require presence of these methods, but if defined, the methods will be called in proper time, see <a href="/5.30.3/perltie">perltie</a>.</p> <p><code>SCALAR</code> is only defined in <b>Tie::StdHash</b> and <b>Tie::ExtraHash</b>.</p> <p>If needed, these methods should be defined by the package inheriting from <b>Tie::Hash</b>, <b>Tie::StdHash</b>, or <b>Tie::ExtraHash</b>. See <a href="/5.30.3/perltie#SCALAR">"SCALAR" in perltie</a> to find out what happens when <code>SCALAR</code> does not exist.</p> <h1 id="MORE-INFORMATION"><a class="permalink" href="#MORE-INFORMATION">#</a><a id="MORE"></a>MORE INFORMATION</h1> <p>The packages relating to various DBM-related implementations (<i>DB_File</i>, <i>NDBM_File</i>, etc.) show examples of general tied hashes, as does the <a href="/5.30.3/Config">Config</a> module. While these do not utilize <b>Tie::Hash</b>, they serve as good working examples.</p> </div> <div id="footer"> <p>Perldoc Browser is maintained by Dan Book (<a href="https://metacpan.org/author/DBOOK">DBOOK</a>). Please contact him via the <a href="https://github.com/Grinnz/perldoc-browser/issues">GitHub issue tracker</a> or <a href="mailto:dbook@cpan.org">email</a> regarding any issues with the site itself, search, or rendering of documentation.</p> <p>The Perl documentation is maintained by the Perl 5 Porters in the development of Perl. 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Tie::Hash, Tie::StdHash, Tie::ExtraHash - base class definitions for tied hashes - Perldoc Browser