tie::memoize(3pm) [redhat man page]
Tie::Memoize(3pm) Perl Programmers Reference Guide Tie::Memoize(3pm) NAME
Tie::Memoize - add data to hash when needed SYNOPSIS
require Tie::Memoize; tie %hash, 'Tie::Memoize', &fetch, # The rest is optional $DATA, &exists, {%ini_value}, {%ini_existence}; DESCRIPTION
This package allows a tied hash to autoload its values on the first access, and to use the cached value on the following accesses. Only read-accesses (via fetching the value or "exists") result in calls to the functions; the modify-accesses are performed as on a normal hash. The required arguments during "tie" are the hash, the package, and the reference to the "FETCH"ing function. The optional arguments are an arbitrary scalar $data, the reference to the "EXISTS" function, and initial values of the hash and of the existence cache. Both the "FETCH"ing function and the "EXISTS" functions have the same signature: the arguments are "$key, $data"; $data is the same value as given as argument during tie()ing. Both functions should return an empty list if the value does not exist. If "EXISTS" function is different from the "FETCH"ing function, it should return a TRUE value on success. The "FETCH"ing function should return the intended value if the key is valid. Inheriting from Tie::Memoize The structure of the tied() data is an array reference with elements 0: cache of known values 1: cache of known existence of keys 2: FETCH function 3: EXISTS function 4: $data The rest is for internal usage of this package. In particular, if TIEHASH is overwritten, it should call SUPER::TIEHASH. EXAMPLE
sub slurp { my ($key, $dir) = shift; open my $h, '<', "$dir/$key" or return; local $/; <$h> # slurp it all } sub exists { my ($key, $dir) = shift; return -f "$dir/$key" } tie %hash, 'Tie::Memoize', &slurp, $directory, &exists, { fake_file1 => $content1, fake_file2 => $content2 }, { pretend_does_not_exists => 0, known_to_exist => 1 }; This example treats the slightly modified contents of $directory as a hash. The modifications are that the keys fake_file1 and fake_file2 fetch values $content1 and $content2, and pretend_does_not_exists will never be accessed. Additionally, the existence of known_to_exist is never checked (so if it does not exists when its content is needed, the user of %hash may be confused). BUGS
FIRSTKEY and NEXTKEY methods go through the keys which were already read, not all the possible keys of the hash. AUTHOR
Ilya Zakharevich <mailto:perl-module-hash-memoize@ilyaz.org>. perl v5.8.0 2002-06-01 Tie::Memoize(3pm)
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Tie::Hash(3pm) Perl Programmers Reference Guide Tie::Hash(3pm) NAME
Tie::Hash, Tie::StdHash, Tie::ExtraHash - base class definitions for tied hashes SYNOPSIS
package NewHash; require Tie::Hash; @ISA = qw(Tie::Hash); sub DELETE { ... } # Provides needed method sub CLEAR { ... } # Overrides inherited method package NewStdHash; require Tie::Hash; @ISA = qw(Tie::StdHash); # All methods provided by default, define only those needing overrides # Accessors access the storage in %{$_[0]}; # TIEHASH should return a reference to the actual storage sub DELETE { ... } package NewExtraHash; require Tie::Hash; @ISA = qw(Tie::ExtraHash); # All methods provided by default, define only those needing overrides # Accessors access the storage in %{$_[0][0]}; # TIEHASH should return an array reference with the first element being # the reference to the actual storage sub DELETE { $_[0][1]->('del', $_[0][0], $_[1]); # Call the report writer delete $_[0][0]->{$_[1]}; # $_[0]->SUPER::DELETE($_[1]) } package main; tie %new_hash, 'NewHash'; tie %new_std_hash, 'NewStdHash'; tie %new_extra_hash, 'NewExtraHash', sub {warn "Doing U$_[1]E of $_[2]. "}; DESCRIPTION
This module provides some skeletal methods for hash-tying classes. See perltie for a list of the functions required in order to tie a hash to a package. The basic Tie::Hash package provides a "new" method, as well as methods "TIEHASH", "EXISTS" and "CLEAR". The Tie::StdHash and Tie::ExtraHash packages provide most methods for hashes described in perltie (the exceptions are "UNTIE" and "DESTROY"). They cause tied hashes to behave exactly like standard hashes, and allow for selective overwriting of methods. Tie::Hash grandfathers the "new" method: it is used if "TIEHASH" is not defined in the case a class forgets to include a "TIEHASH" method. For developers wishing to write their own tied hashes, the required methods are briefly defined below. See the perltie section for more detailed descriptive, as well as example code: TIEHASH classname, LIST The method invoked by the command "tie %hash, classname". Associates a new hash instance with the specified class. "LIST" would represent additional arguments (along the lines of AnyDBM_File and compatriots) needed to complete the association. STORE this, key, value Store datum value into key for the tied hash this. FETCH this, key Retrieve the datum in key for the tied hash this. FIRSTKEY this Return the first key in the hash. NEXTKEY this, lastkey Return the next key in the hash. EXISTS this, key Verify that key exists with the tied hash this. The Tie::Hash implementation is a stub that simply croaks. DELETE this, key Delete the key key from the tied hash this. CLEAR this Clear all values from the tied hash this. SCALAR this Returns what evaluating the hash in scalar context yields. Tie::Hash does not implement this method (but Tie::StdHash and Tie::ExtraHash do). Inheriting from Tie::StdHash The accessor methods assume that the actual storage for the data in the tied hash is in the hash referenced by "tied(%tiedhash)". Thus overwritten "TIEHASH" method should return a hash reference, and the remaining methods should operate on the hash referenced by the first argument: package ReportHash; our @ISA = 'Tie::StdHash'; sub TIEHASH { my $storage = bless {}, shift; warn "New ReportHash created, stored in $storage. "; $storage } sub STORE { warn "Storing data with key $_[1] at $_[0]. "; $_[0]{$_[1]} = $_[2] } Inheriting from Tie::ExtraHash The accessor methods assume that the actual storage for the data in the tied hash is in the hash referenced by "(tied(%tiedhash))->[0]". Thus overwritten "TIEHASH" method should return an array reference with the first element being a hash reference, and the remaining methods should operate on the hash "%{ $_[0]->[0] }": package ReportHash; our @ISA = 'Tie::ExtraHash'; sub TIEHASH { my $class = shift; my $storage = bless [{}, @_], $class; warn "New ReportHash created, stored in $storage. "; $storage; } sub STORE { warn "Storing data with key $_[1] at $_[0]. "; $_[0][0]{$_[1]} = $_[2] } The default "TIEHASH" method stores "extra" arguments to tie() starting from offset 1 in the array referenced by "tied(%tiedhash)"; this is the same storage algorithm as in TIEHASH subroutine above. Hence, a typical package inheriting from Tie::ExtraHash does not need to overwrite this method. "SCALAR", "UNTIE" and "DESTROY" The methods "UNTIE" and "DESTROY" are not defined in Tie::Hash, Tie::StdHash, or Tie::ExtraHash. Tied hashes do not require presence of these methods, but if defined, the methods will be called in proper time, see perltie. "SCALAR" is only defined in Tie::StdHash and Tie::ExtraHash. If needed, these methods should be defined by the package inheriting from Tie::Hash, Tie::StdHash, or Tie::ExtraHash. See "SCALAR" in perltie to find out what happens when "SCALAR" does not exist. MORE INFORMATION
The packages relating to various DBM-related implementations (DB_File, NDBM_File, etc.) show examples of general tied hashes, as does the Config module. While these do not utilize Tie::Hash, they serve as good working examples. perl v5.18.2 2014-01-06 Tie::Hash(3pm)