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PERLOBJ(1) User Contributed Perl Documentation PERLOBJ(1) NNNNAAAAMMMMEEEE perlobj - Perl objects DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN First of all, you need to understand what references are in Perl. See the _p_e_r_l_r_e_f manpage for that. Here are three very simple definitions that you should find reassuring. 1. An object is simply a reference that happens to know which class it belongs to. 2. A class is simply a package that happens to provide methods to deal with object references. 3. A method is simply a subroutine that expects an object reference (or a package name, for static methods) as the first argument. We'll cover these points now in more depth. AAAAnnnn OOOObbbbjjjjeeeecccctttt iiiissss SSSSiiiimmmmppppllllyyyy aaaa RRRReeeeffffeeeerrrreeeennnncccceeee Unlike say C++, Perl doesn't provide any special syntax for constructors. A constructor is merely a subroutine that returns a reference to something "blessed" into a class, generally the class that the subroutine is defined in. Here is a typical constructor: ppppaaaacccckkkkaaaaggggeeee CCCCrrrriiiitttttttteeeerrrr;;;; ssssuuuubbbb nnnneeeewwww {{{{ bbbblllleeeessssssss {{{{}}}} }}}} The {{{{}}}} constructs a reference to an anonymous hash containing no key/value pairs. The _b_l_e_s_s_(_) takes that reference and tells the object it references that it's now a Critter, and returns the reference. This is for convenience, since the referenced object itself knows that it has been blessed, and its reference to it could have been returned directly, like this: ssssuuuubbbb nnnneeeewwww {{{{ mmmmyyyy $$$$sssseeeellllffff ==== {{{{}}}};;;; bbbblllleeeessssssss $$$$sssseeeellllffff;;;; rrrreeeettttuuuurrrrnnnn $$$$sssseeeellllffff;;;; }}}} In fact, you often see such a thing in more complicated constructors that wish to call methods in the class as part of the construction: 23/Jan/96 perl 5.002 with 1 PERLOBJ(1) User Contributed Perl Documentation PERLOBJ(1) ssssuuuubbbb nnnneeeewwww {{{{ mmmmyyyy $$$$sssseeeellllffff ==== {{{{}}}} bbbblllleeeessssssss $$$$sssseeeellllffff;;;; $$$$sssseeeellllffff---->>>>iiiinnnniiiittttiiiiaaaalllliiiizzzzeeee(((())));;;; rrrreeeettttuuuurrrrnnnn $$$$sssseeeellllffff;;;; }}}} If you care about inheritance (and you should; see L<perlmod/"Modules: Creation, Use and Abuse">), then you want to use the two-arg form of bless so that your constructors may be inherited: ssssuuuubbbb nnnneeeewwww {{{{ mmmmyyyy $$$$ccccllllaaaassssssss ==== sssshhhhiiiifffftttt;;;; mmmmyyyy $$$$sssseeeellllffff ==== {{{{}}}};;;; bbbblllleeeessssssss $$$$sssseeeellllffff,,,, $$$$ccccllllaaaassssssss $$$$sssseeeellllffff---->>>>iiiinnnniiiittttiiiiaaaalllliiiizzzzeeee(((())));;;; rrrreeeettttuuuurrrrnnnn $$$$sssseeeellllffff;;;; }}}} Or if you expect people to call not just CCCCLLLLAAAASSSSSSSS----_n_e_w_(_)> but also $$$$oooobbbbjjjj----_n_e_w_(_)>, then use something like this. The _i_n_i_t_i_a_l_i_z_e_(_) method used will be of whatever $$$$ccccllllaaaassssssss we blessed the object into: ssssuuuubbbb nnnneeeewwww {{{{ mmmmyyyy $$$$tttthhhhiiiissss ==== sssshhhhiiiifffftttt;;;; mmmmyyyy $$$$ccccllllaaaassssssss ==== rrrreeeeffff(((($$$$tttthhhhiiiissss)))) |||||||| $$$$tttthhhhiiiissss;;;; mmmmyyyy $$$$sssseeeellllffff ==== {{{{}}}};;;; bbbblllleeeessssssss $$$$sssseeeellllffff,,,, $$$$ccccllllaaaassssssss $$$$sssseeeellllffff---->>>>iiiinnnniiiittttiiiiaaaalllliiiizzzzeeee(((())));;;; rrrreeeettttuuuurrrrnnnn $$$$sssseeeellllffff;;;; }}}} Within the class package, the methods will typically deal with the reference as an ordinary reference. Outside the class package, the reference is generally treated as an opaque value that may only be accessed through the class's methods. A constructor may re-bless a referenced object currently belonging to another class, but then the new class is responsible for all cleanup later. The previous blessing is forgotten, as an object may only belong to one class at a time. (Although of course it's free to inherit methods from many classes.) A clarification: Perl objects are blessed. References are not. Objects know which package they belong to. References do not. The _b_l_e_s_s_(_) function simply uses the reference in order to find the object. Consider the following example: 23/Jan/96 perl 5.002 with 2 PERLOBJ(1) User Contributed Perl Documentation PERLOBJ(1) $$$$aaaa ==== {{{{}}}};;;; $$$$bbbb ==== $$$$aaaa;;;; bbbblllleeeessssssss $$$$aaaa,,,, BBBBLLLLAAAAHHHH;;;; pppprrrriiiinnnntttt """"\\\\$$$$bbbb iiiissss aaaa """",,,, rrrreeeeffff(((($$$$bbbb)))),,,, """"\\\\nnnn"""";;;; This reports $$$$bbbb as being a BLAH, so obviously _b_l_e_s_s_(_) operated on the object and not on the reference. AAAA CCCCllllaaaassssssss iiiissss SSSSiiiimmmmppppllllyyyy aaaa PPPPaaaacccckkkkaaaaggggeeee Unlike say C++, Perl doesn't provide any special syntax for class definitions. You just use a package as a class by putting method definitions into the class. There is a special array within each package called @@@@IIIISSSSAAAA which says where else to look for a method if you can't find it in the current package. This is how Perl implements inheritance. Each element of the @@@@IIIISSSSAAAA array is just the name of another package that happens to be a class package. The classes are searched (depth first) for missing methods in the order that they occur in @@@@IIIISSSSAAAA. The classes accessible through @@@@IIIISSSSAAAA are known as base classes of the current class. If a missing method is found in one of the base classes, it is cached in the current class for efficiency. Changing @@@@IIIISSSSAAAA or defining new subroutines invalidates the cache and causes Perl to do the lookup again. If a method isn't found, but an AUTOLOAD routine is found, then that is called on behalf of the missing method. If neither a method nor an AUTOLOAD routine is found in @@@@IIIISSSSAAAA, then one last try is made for the method (or an AUTOLOAD routine) in a class called UNIVERSAL. If that doesn't work, Perl finally gives up and complains. Perl classes only do method inheritance. Data inheritance is left up to the class itself. By and large, this is not a problem in Perl, because most classes model the attributes of their object using an anonymous hash, which serves as its own little namespace to be carved up by the various classes that might want to do something with the object. AAAA MMMMeeeetttthhhhoooodddd iiiissss SSSSiiiimmmmppppllllyyyy aaaa SSSSuuuubbbbrrrroooouuuuttttiiiinnnneeee Unlike say C++, Perl doesn't provide any special syntax for method definition. (It does provide a little syntax for method invocation though. More on that later.) A method expects its first argument to be the object or package it is being invoked on. There are just two types of methods, which we'll call static and virtual, in honor of the two C++ method types they most closely resemble. 23/Jan/96 perl 5.002 with 3 PERLOBJ(1) User Contributed Perl Documentation PERLOBJ(1) A static method expects a class name as the first argument. It provides functionality for the class as a whole, not for any individual object belonging to the class. Constructors are typically static methods. Many static methods simply ignore their first argument, since they already know what package they're in, and don't care what package they were invoked via. (These aren't necessarily the same, since static methods follow the inheritance tree just like ordinary virtual methods.) Another typical use for static methods is to look up an object by name: ssssuuuubbbb ffffiiiinnnndddd {{{{ mmmmyyyy (((($$$$ccccllllaaaassssssss,,,, $$$$nnnnaaaammmmeeee)))) ==== @@@@____;;;; $$$$oooobbbbjjjjttttaaaabbbblllleeee{{{{$$$$nnnnaaaammmmeeee}}}};;;; }}}} A virtual method expects an object reference as its first argument. Typically it shifts the first argument into a "self" or "this" variable, and then uses that as an ordinary reference. ssssuuuubbbb ddddiiiissssppppllllaaaayyyy {{{{ mmmmyyyy $$$$sssseeeellllffff ==== sssshhhhiiiifffftttt;;;; mmmmyyyy @@@@kkkkeeeeyyyyssss ==== @@@@____ ???? @@@@____ :::: ssssoooorrrrtttt kkkkeeeeyyyyssss %%%%$$$$sssseeeellllffff;;;; ffffoooorrrreeeeaaaacccchhhh $$$$kkkkeeeeyyyy ((((@@@@kkkkeeeeyyyyssss)))) {{{{ pppprrrriiiinnnntttt """"\\\\tttt$$$$kkkkeeeeyyyy ====>>>> $$$$sssseeeellllffff---->>>>{{{{$$$$kkkkeeeeyyyy}}}}\\\\nnnn"""";;;; }}}} }}}} MMMMeeeetttthhhhoooodddd IIIInnnnvvvvooooccccaaaattttiiiioooonnnn There are two ways to invoke a method, one of which you're already familiar with, and the other of which will look familiar. Perl 4 already had an "indirect object" syntax that you use when you say pppprrrriiiinnnntttt SSSSTTTTDDDDEEEERRRRRRRR """"hhhheeeellllpppp!!!!!!!!!!!!\\\\nnnn"""";;;; This same syntax can be used to call either static or virtual methods. We'll use the two methods defined above, the static method to lookup an object reference and the virtual method to print out its attributes. $$$$ffffrrrreeeedddd ==== ffffiiiinnnndddd CCCCrrrriiiitttttttteeeerrrr """"FFFFrrrreeeedddd"""";;;; ddddiiiissssppppllllaaaayyyy $$$$ffffrrrreeeedddd ''''HHHHeeeeiiiigggghhhhtttt'''',,,, ''''WWWWeeeeiiiigggghhhhtttt'''';;;; These could be combined into one statement by using a BLOCK in the indirect object slot: ddddiiiissssppppllllaaaayyyy {{{{ffffiiiinnnndddd CCCCrrrriiiitttttttteeeerrrr """"FFFFrrrreeeedddd""""}}}} ''''HHHHeeeeiiiigggghhhhtttt'''',,,, ''''WWWWeeeeiiiigggghhhhtttt'''';;;; For C++ fans, there's also a syntax using -> notation that 23/Jan/96 perl 5.002 with 4 PERLOBJ(1) User Contributed Perl Documentation PERLOBJ(1) does exactly the same thing. The parentheses are required if there are any arguments. $$$$ffffrrrreeeedddd ==== CCCCrrrriiiitttttttteeeerrrr---->>>>ffffiiiinnnndddd((((""""FFFFrrrreeeedddd""""))));;;; $$$$ffffrrrreeeedddd---->>>>ddddiiiissssppppllllaaaayyyy((((''''HHHHeeeeiiiigggghhhhtttt'''',,,, ''''WWWWeeeeiiiigggghhhhtttt''''))));;;; or in one statement, CCCCrrrriiiitttttttteeeerrrr---->>>>ffffiiiinnnndddd((((""""FFFFrrrreeeedddd""""))))---->>>>ddddiiiissssppppllllaaaayyyy((((''''HHHHeeeeiiiigggghhhhtttt'''',,,, ''''WWWWeeeeiiiigggghhhhtttt''''))));;;; There are times when one syntax is more readable, and times when the other syntax is more readable. The indirect object syntax is less cluttered, but it has the same ambiguity as ordinary list operators. Indirect object method calls are parsed using the same rule as list operators: "If it looks like a function, it is a function". (Presuming for the moment that you think two words in a row can look like a function name. C++ programmers seem to think so with some regularity, especially when the first word is "new".) Thus, the parens of nnnneeeewwww CCCCrrrriiiitttttttteeeerrrr ((((''''BBBBaaaarrrrnnnneeeeyyyy'''',,,, 1111....5555,,,, 77770000)))) are assumed to surround ALL the arguments of the method call, regardless of what comes after. Saying nnnneeeewwww CCCCrrrriiiitttttttteeeerrrr ((((''''BBBBaaaammmm'''' xxxx 2222)))),,,, 1111....4444,,,, 44445555 would be equivalent to CCCCrrrriiiitttttttteeeerrrr---->>>>nnnneeeewwww((((''''BBBBaaaammmm'''' xxxx 2222)))),,,, 1111....4444,,,, 44445555 which is unlikely to do what you want. There are times when you wish to specify which class's method to use. In this case, you can call your method as an ordinary subroutine call, being sure to pass the requisite first argument explicitly: $$$$ffffrrrreeeedddd ==== MMMMyyyyCCCCrrrriiiitttttttteeeerrrr::::::::ffffiiiinnnndddd((((""""CCCCrrrriiiitttttttteeeerrrr"""",,,, """"FFFFrrrreeeedddd""""))));;;; MMMMyyyyCCCCrrrriiiitttttttteeeerrrr::::::::ddddiiiissssppppllllaaaayyyy(((($$$$ffffrrrreeeedddd,,,, ''''HHHHeeeeiiiigggghhhhtttt'''',,,, ''''WWWWeeeeiiiigggghhhhtttt''''))));;;; Note however, that this does not do any inheritance. If you merely wish to specify that Perl should _S_T_A_R_T looking for a method in a particular package, use an ordinary method call, but qualify the method name with the package like this: $$$$ffffrrrreeeedddd ==== CCCCrrrriiiitttttttteeeerrrr---->>>>MMMMyyyyCCCCrrrriiiitttttttteeeerrrr::::::::ffffiiiinnnndddd((((""""FFFFrrrreeeedddd""""))));;;; $$$$ffffrrrreeeedddd---->>>>MMMMyyyyCCCCrrrriiiitttttttteeeerrrr::::::::ddddiiiissssppppllllaaaayyyy((((''''HHHHeeeeiiiigggghhhhtttt'''',,,, ''''WWWWeeeeiiiigggghhhhtttt''''))));;;; If you're trying to control where the method search begins _a_n_d you're executing in the class itself, then you may use 23/Jan/96 perl 5.002 with 5 PERLOBJ(1) User Contributed Perl Documentation PERLOBJ(1) the SUPER pseudoclass, which says to start looking in your base class's @@@@IIIISSSSAAAA list without having to explicitly name it: $$$$sssseeeellllffff---->>>>SSSSUUUUPPPPEEEERRRR::::::::ddddiiiissssppppllllaaaayyyy((((''''HHHHeeeeiiiigggghhhhtttt'''',,,, ''''WWWWeeeeiiiigggghhhhtttt''''))));;;; Please note that the SSSSUUUUPPPPEEEERRRR:::::::: construct is _o_n_l_y meaningful within the class. Sometimes you want to call a method when you don't know the method name ahead of time. You can use the arrow form, replacing the method name with a simple scalar variable containing the method name: $$$$mmmmeeeetttthhhhoooodddd ==== $$$$ffffaaaasssstttt ???? """"ffffiiiinnnnddddffffiiiirrrrsssstttt"""" :::: """"ffffiiiinnnnddddbbbbeeeesssstttt"""";;;; $$$$ffffrrrreeeedddd---->>>>$$$$mmmmeeeetttthhhhoooodddd((((@@@@aaaarrrrggggssss))));;;; DDDDeeeessssttttrrrruuuuccccttttoooorrrrssss When the last reference to an object goes away, the object is automatically destroyed. (This may even be after you exit, if you've stored references in global variables.) If you want to capture control just before the object is freed, you may define a DESTROY method in your class. It will automatically be called at the appropriate moment, and you can do any extra cleanup you need to do. Perl doesn't do nested destruction for you. If your constructor reblessed a reference from one of your base classes, your DESTROY may need to call DESTROY for any base classes that need it. But this only applies to reblessed objects--an object reference that is merely _C_O_N_T_A_I_N_E_D in the current object will be freed and destroyed automatically when the current object is freed. WWWWAAAARRRRNNNNIIIINNNNGGGG An indirect object is limited to a name, a scalar variable, or a block, because it would have to do too much lookahead otherwise, just like any other postfix dereference in the language. The left side of -> is not so limited, because it's an infix operator, not a postfix operator. That means that below, A and B are equivalent to each other, and C and D are equivalent, but AB and CD are different: AAAA:::: mmmmeeeetttthhhhoooodddd $$$$oooobbbbrrrreeeeffff---->>>>{{{{""""ffffiiiieeeellllddddnnnnaaaammmmeeee""""}}}} BBBB:::: ((((mmmmeeeetttthhhhoooodddd $$$$oooobbbbrrrreeeeffff))))---->>>>{{{{""""ffffiiiieeeellllddddnnnnaaaammmmeeee""""}}}} CCCC:::: $$$$oooobbbbrrrreeeeffff---->>>>{{{{""""ffffiiiieeeellllddddnnnnaaaammmmeeee""""}}}}---->>>>mmmmeeeetttthhhhoooodddd(((()))) DDDD:::: mmmmeeeetttthhhhoooodddd {{{{$$$$oooobbbbrrrreeeeffff---->>>>{{{{""""ffffiiiieeeellllddddnnnnaaaammmmeeee""""}}}}}}}} 23/Jan/96 perl 5.002 with 6 PERLOBJ(1) User Contributed Perl Documentation PERLOBJ(1) SSSSuuuummmmmmmmaaaarrrryyyy That's about all there is to it. Now you just need to go off and buy a book about object-oriented design methodology, and bang your forehead with it for the next six months or so. TTTTwwwwoooo----PPPPhhhhaaaasssseeeedddd GGGGaaaarrrrbbbbaaaaggggeeee CCCCoooolllllllleeeeccccttttiiiioooonnnn For most purposes, Perl uses a fast and simple reference- based garbage collection system. For this reason, there's an extra dereference going on at some level, so if you haven't built your Perl executable using your C compiler's ----OOOO flag, performance will suffer. If you _h_a_v_e built Perl with cccccccc ----OOOO, then this probably won't matter. A more serious concern is that unreachable memory with a non-zero reference count will not normally get freed. Therefore, this is a bad idea: {{{{ mmmmyyyy $$$$aaaa;;;; $$$$aaaa ==== \\\\$$$$aaaa;;;; }}}} Even thought $$$$aaaa _s_h_o_u_l_d go away, it can't. When building recursive data structures, you'll have to break the self- reference yourself explicitly if you don't care to leak. For example, here's a self-referential node such as one might use in a sophisticated tree structure: ssssuuuubbbb nnnneeeewwww____nnnnooooddddeeee {{{{ mmmmyyyy $$$$sssseeeellllffff ==== sssshhhhiiiifffftttt;;;; mmmmyyyy $$$$ccccllllaaaassssssss ==== rrrreeeeffff(((($$$$sssseeeellllffff)))) |||||||| $$$$sssseeeellllffff;;;; mmmmyyyy $$$$nnnnooooddddeeee ==== {{{{}}}};;;; $$$$nnnnooooddddeeee---->>>>{{{{LLLLEEEEFFFFTTTT}}}} ==== $$$$nnnnooooddddeeee---->>>>{{{{RRRRIIIIGGGGHHHHTTTT}}}} ==== $$$$nnnnooooddddeeee;;;; $$$$nnnnooooddddeeee---->>>>{{{{DDDDAAAATTTTAAAA}}}} ==== [[[[ @@@@____ ]]]];;;; rrrreeeettttuuuurrrrnnnn bbbblllleeeessssssss $$$$nnnnooooddddeeee ====>>>> $$$$ccccllllaaaassssssss;;;; }}}} If you create nodes like that, they (currently) won't go away unless you break their self reference yourself. (In other words, this is not to be construed as a feature, and you shouldn't depend on it.) Almost. When an interpreter thread finally shuts down (usually when your program exits), then a rather costly but complete mark-and-sweep style of garbage collection is performed, and everything allocated by that thread gets destroyed. This is essential to support Perl as an embedded or a multithreadable language. For example, this program demonstrates Perl's two-phased garbage collection: 23/Jan/96 perl 5.002 with 7 PERLOBJ(1) User Contributed Perl Documentation PERLOBJ(1) ####!!!!////uuuussssrrrr////bbbbiiiinnnn////ppppeeeerrrrllll ppppaaaacccckkkkaaaaggggeeee SSSSuuuubbbbttttlllleeee;;;; ssssuuuubbbb nnnneeeewwww {{{{ mmmmyyyy $$$$tttteeeesssstttt;;;; $$$$tttteeeesssstttt ==== \\\\$$$$tttteeeesssstttt;;;; wwwwaaaarrrrnnnn """"CCCCRRRREEEEAAAATTTTIIIINNNNGGGG """" .... \\\\$$$$tttteeeesssstttt;;;; rrrreeeettttuuuurrrrnnnn bbbblllleeeessssssss \\\\$$$$tttteeeesssstttt;;;; }}}} ssssuuuubbbb DDDDEEEESSSSTTTTRRRROOOOYYYY {{{{ mmmmyyyy $$$$sssseeeellllffff ==== sssshhhhiiiifffftttt;;;; wwwwaaaarrrrnnnn """"DDDDEEEESSSSTTTTRRRROOOOYYYYIIIINNNNGGGG $$$$sssseeeellllffff"""";;;; }}}} ppppaaaacccckkkkaaaaggggeeee mmmmaaaaiiiinnnn;;;; wwwwaaaarrrrnnnn """"ssssttttaaaarrrrttttiiiinnnngggg pppprrrrooooggggrrrraaaammmm"""";;;; {{{{ mmmmyyyy $$$$aaaa ==== SSSSuuuubbbbttttlllleeee---->>>>nnnneeeewwww;;;; mmmmyyyy $$$$bbbb ==== SSSSuuuubbbbttttlllleeee---->>>>nnnneeeewwww;;;; $$$$$$$$aaaa ==== 0000;;;; #### bbbbrrrreeeeaaaakkkk sssseeeellllffffrrrreeeeffff wwwwaaaarrrrnnnn """"lllleeeeaaaavvvviiiinnnngggg bbbblllloooocccckkkk"""";;;; }}}} wwwwaaaarrrrnnnn """"jjjjuuuusssstttt eeeexxxxiiiitttteeeedddd bbbblllloooocccckkkk"""";;;; wwwwaaaarrrrnnnn """"ttttiiiimmmmeeee ttttoooo ddddiiiieeee............"""";;;; eeeexxxxiiiitttt;;;; When run as _/_t_m_p_/_t_e_s_t, the following output is produced: ssssttttaaaarrrrttttiiiinnnngggg pppprrrrooooggggrrrraaaammmm aaaatttt ////ttttmmmmpppp////tttteeeesssstttt lllliiiinnnneeee 11118888.... CCCCRRRREEEEAAAATTTTIIIINNNNGGGG SSSSCCCCAAAALLLLAAAARRRR((((0000xxxx8888eeee5555bbbb8888)))) aaaatttt ////ttttmmmmpppp////tttteeeesssstttt lllliiiinnnneeee 7777.... CCCCRRRREEEEAAAATTTTIIIINNNNGGGG SSSSCCCCAAAALLLLAAAARRRR((((0000xxxx8888eeee55557777cccc)))) aaaatttt ////ttttmmmmpppp////tttteeeesssstttt lllliiiinnnneeee 7777.... lllleeeeaaaavvvviiiinnnngggg bbbblllloooocccckkkk aaaatttt ////ttttmmmmpppp////tttteeeesssstttt lllliiiinnnneeee 22223333.... DDDDEEEESSSSTTTTRRRROOOOYYYYIIIINNNNGGGG SSSSuuuubbbbttttlllleeee====SSSSCCCCAAAALLLLAAAARRRR((((0000xxxx8888eeee5555bbbb8888)))) aaaatttt ////ttttmmmmpppp////tttteeeesssstttt lllliiiinnnneeee 11113333.... jjjjuuuusssstttt eeeexxxxiiiitttteeeedddd bbbblllloooocccckkkk aaaatttt ////ttttmmmmpppp////tttteeeesssstttt lllliiiinnnneeee 22226666.... ttttiiiimmmmeeee ttttoooo ddddiiiieeee............ aaaatttt ////ttttmmmmpppp////tttteeeesssstttt lllliiiinnnneeee 22227777.... DDDDEEEESSSSTTTTRRRROOOOYYYYIIIINNNNGGGG SSSSuuuubbbbttttlllleeee====SSSSCCCCAAAALLLLAAAARRRR((((0000xxxx8888eeee55557777cccc)))) dddduuuurrrriiiinnnngggg gggglllloooobbbbaaaallll ddddeeeessssttttrrrruuuuccccttttiiiioooonnnn.... Notice that "global destruction" bit there? That's the thread garbage collector reaching the unreachable. Objects are always destructed, even when regular refs aren't and in fact are destructed in a separate pass before ordinary refs just to try to prevent object destructors from using refs that have been themselves destructed. Plain refs are only garbage collected if the destruct level is greater than 0. You can test the higher levels of global destruction by setting the PERL_DESTRUCT_LEVEL environment variable, presuming ----DDDDDDDDEEEEBBBBUUUUGGGGGGGGIIIINNNNGGGG was enabled during perl build time. A more complete garbage collection strategy will be 23/Jan/96 perl 5.002 with 8 PERLOBJ(1) User Contributed Perl Documentation PERLOBJ(1) implemented at a future date. SSSSEEEEEEEE AAAALLLLSSSSOOOO You should also check out the _p_e_r_l_b_o_t manpage for other object tricks, traps, and tips, as well as the _p_e_r_l_m_o_d manpage for some style guides on constructing both modules and classes. 23/Jan/96 perl 5.002 with 9