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PERLBOT(1) User Contributed Perl Documentation PERLBOT(1) NNNNAAAAMMMMEEEE perlbot - Bag'o Object Tricks (the BOT) DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN The following collection of tricks and hints is intended to whet curious appetites about such things as the use of instance variables and the mechanics of object and class relationships. The reader is encouraged to consult relevant textbooks for discussion of Object Oriented definitions and methodology. This is not intended as a tutorial for object-oriented programming or as a comprehensive guide to Perl's object oriented features, nor should it be construed as a style guide. The Perl motto still holds: There's more than one way to do it. OOOOOOOO SSSSCCCCAAAALLLLIIIINNNNGGGG TTTTIIIIPPPPSSSS 1 Do not attempt to verify the type of $$$$sssseeeellllffff. That'll break if the class is inherited, when the type of $$$$sssseeeellllffff is valid but its package isn't what you expect. See rule 5. 2 If an object-oriented (OO) or indirect-object (IO) syntax was used, then the object is probably the correct type and there's no need to become paranoid about it. Perl isn't a paranoid language anyway. If people subvert the OO or IO syntax then they probably know what they're doing and you should let them do it. See rule 1. 3 Use the two-argument form of _b_l_e_s_s_(_). Let a subclass use your constructor. See the section on _I_N_H_E_R_I_T_I_N_G _A _C_O_N_S_T_R_U_C_T_O_R. 4 The subclass is allowed to know things about its immediate superclass, the superclass is allowed to know nothing about a subclass. 5 Don't be trigger happy with inheritance. A "using", "containing", or "delegation" relationship (some sort of aggregation, at least) is often more appropriate. See the section on _O_B_J_E_C_T _R_E_L_A_T_I_O_N_S_H_I_P_S, the section on _U_S_I_N_G _R_E_L_A_T_I_O_N_S_H_I_P _W_I_T_H _S_D_B_M, and the section on _D_E_L_E_G_A_T_I_O_N. 6 The object is the namespace. Make package globals accessible via the object. This will remove the guess work about the symbol's home package. See the section on _C_L_A_S_S _C_O_N_T_E_X_T _A_N_D _T_H_E _O_B_J_E_C_T. 7 IO syntax is certainly less noisy, but it is also prone to ambiguities which can cause difficult-to- find bugs. Allow people to use the sure-thing OO 12/Feb/96 perl 5.002 with 1 PERLBOT(1) User Contributed Perl Documentation PERLBOT(1) syntax, even if you don't like it. 8 Do not use function-call syntax on a method. You're going to be bitten someday. Someone might move that method into a superclass and your code will be broken. On top of that you're feeding the paranoia in rule 2. 9 Don't assume you know the home package of a method. You're making it difficult for someone to override that method. See the section on _T_H_I_N_K_I_N_G _O_F _C_O_D_E _R_E_U_S_E. IIIINNNNSSSSTTTTAAAANNNNCCCCEEEE VVVVAAAARRRRIIIIAAAABBBBLLLLEEEESSSS An anonymous array or anonymous hash can be used to hold instance variables. Named parameters are also demonstrated. ppppaaaacccckkkkaaaaggggeeee FFFFoooooooo;;;; ssssuuuubbbb nnnneeeewwww {{{{ mmmmyyyy $$$$ttttyyyyppppeeee ==== sssshhhhiiiifffftttt;;;; mmmmyyyy %%%%ppppaaaarrrraaaammmmssss ==== @@@@____;;;; mmmmyyyy $$$$sssseeeellllffff ==== {{{{}}}};;;; $$$$sssseeeellllffff---->>>>{{{{''''HHHHiiiigggghhhh''''}}}} ==== $$$$ppppaaaarrrraaaammmmssss{{{{''''HHHHiiiigggghhhh''''}}}};;;; $$$$sssseeeellllffff---->>>>{{{{''''LLLLoooowwww''''}}}} ==== $$$$ppppaaaarrrraaaammmmssss{{{{''''LLLLoooowwww''''}}}};;;; bbbblllleeeessssssss $$$$sssseeeellllffff,,,, $$$$ttttyyyyppppeeee;;;; }}}} ppppaaaacccckkkkaaaaggggeeee BBBBaaaarrrr;;;; ssssuuuubbbb nnnneeeewwww {{{{ mmmmyyyy $$$$ttttyyyyppppeeee ==== sssshhhhiiiifffftttt;;;; mmmmyyyy %%%%ppppaaaarrrraaaammmmssss ==== @@@@____;;;; mmmmyyyy $$$$sssseeeellllffff ==== [[[[]]]];;;; $$$$sssseeeellllffff---->>>>[[[[0000]]]] ==== $$$$ppppaaaarrrraaaammmmssss{{{{''''LLLLeeeefffftttt''''}}}};;;; $$$$sssseeeellllffff---->>>>[[[[1111]]]] ==== $$$$ppppaaaarrrraaaammmmssss{{{{''''RRRRiiiigggghhhhtttt''''}}}};;;; bbbblllleeeessssssss $$$$sssseeeellllffff,,,, $$$$ttttyyyyppppeeee;;;; }}}} ppppaaaacccckkkkaaaaggggeeee mmmmaaaaiiiinnnn;;;; $$$$aaaa ==== FFFFoooooooo---->>>>nnnneeeewwww(((( ''''HHHHiiiigggghhhh'''' ====>>>> 44442222,,,, ''''LLLLoooowwww'''' ====>>>> 11111111 ))));;;; pppprrrriiiinnnntttt """"HHHHiiiigggghhhh====$$$$aaaa---->>>>{{{{''''HHHHiiiigggghhhh''''}}}}\\\\nnnn"""";;;; pppprrrriiiinnnntttt """"LLLLoooowwww====$$$$aaaa---->>>>{{{{''''LLLLoooowwww''''}}}}\\\\nnnn"""";;;; $$$$bbbb ==== BBBBaaaarrrr---->>>>nnnneeeewwww(((( ''''LLLLeeeefffftttt'''' ====>>>> 77778888,,,, ''''RRRRiiiigggghhhhtttt'''' ====>>>> 44440000 ))));;;; pppprrrriiiinnnntttt """"LLLLeeeefffftttt====$$$$bbbb---->>>>[[[[0000]]]]\\\\nnnn"""";;;; pppprrrriiiinnnntttt """"RRRRiiiigggghhhhtttt====$$$$bbbb---->>>>[[[[1111]]]]\\\\nnnn"""";;;; SSSSCCCCAAAALLLLAAAARRRR IIIINNNNSSSSTTTTAAAANNNNCCCCEEEE VVVVAAAARRRRIIIIAAAABBBBLLLLEEEESSSS An anonymous scalar can be used when only one instance variable is needed. 12/Feb/96 perl 5.002 with 2 PERLBOT(1) User Contributed Perl Documentation PERLBOT(1) ppppaaaacccckkkkaaaaggggeeee FFFFoooooooo;;;; ssssuuuubbbb nnnneeeewwww {{{{ mmmmyyyy $$$$ttttyyyyppppeeee ==== sssshhhhiiiifffftttt;;;; mmmmyyyy $$$$sssseeeellllffff;;;; $$$$sssseeeellllffff ==== sssshhhhiiiifffftttt;;;; bbbblllleeeessssssss \\\\$$$$sssseeeellllffff,,,, $$$$ttttyyyyppppeeee;;;; }}}} ppppaaaacccckkkkaaaaggggeeee mmmmaaaaiiiinnnn;;;; $$$$aaaa ==== FFFFoooooooo---->>>>nnnneeeewwww(((( 44442222 ))));;;; pppprrrriiiinnnntttt """"aaaa====$$$$$$$$aaaa\\\\nnnn"""";;;; IIIINNNNSSSSTTTTAAAANNNNCCCCEEEE VVVVAAAARRRRIIIIAAAABBBBLLLLEEEE IIIINNNNHHHHEEEERRRRIIIITTTTAAAANNNNCCCCEEEE This example demonstrates how one might inherit instance variables from a superclass for inclusion in the new class. This requires calling the superclass's constructor and adding one's own instance variables to the new object. ppppaaaacccckkkkaaaaggggeeee BBBBaaaarrrr;;;; ssssuuuubbbb nnnneeeewwww {{{{ mmmmyyyy $$$$ttttyyyyppppeeee ==== sssshhhhiiiifffftttt;;;; mmmmyyyy $$$$sssseeeellllffff ==== {{{{}}}};;;; $$$$sssseeeellllffff---->>>>{{{{''''bbbbuuuuzzzz''''}}}} ==== 44442222;;;; bbbblllleeeessssssss $$$$sssseeeellllffff,,,, $$$$ttttyyyyppppeeee;;;; }}}} ppppaaaacccckkkkaaaaggggeeee FFFFoooooooo;;;; @@@@IIIISSSSAAAA ==== qqqqwwww(((( BBBBaaaarrrr ))));;;; ssssuuuubbbb nnnneeeewwww {{{{ mmmmyyyy $$$$ttttyyyyppppeeee ==== sssshhhhiiiifffftttt;;;; mmmmyyyy $$$$sssseeeellllffff ==== BBBBaaaarrrr---->>>>nnnneeeewwww;;;; $$$$sssseeeellllffff---->>>>{{{{''''bbbbiiiizzzz''''}}}} ==== 11111111;;;; bbbblllleeeessssssss $$$$sssseeeellllffff,,,, $$$$ttttyyyyppppeeee;;;; }}}} ppppaaaacccckkkkaaaaggggeeee mmmmaaaaiiiinnnn;;;; $$$$aaaa ==== FFFFoooooooo---->>>>nnnneeeewwww;;;; pppprrrriiiinnnntttt """"bbbbuuuuzzzz ==== """",,,, $$$$aaaa---->>>>{{{{''''bbbbuuuuzzzz''''}}}},,,, """"\\\\nnnn"""";;;; pppprrrriiiinnnntttt """"bbbbiiiizzzz ==== """",,,, $$$$aaaa---->>>>{{{{''''bbbbiiiizzzz''''}}}},,,, """"\\\\nnnn"""";;;; OOOOBBBBJJJJEEEECCCCTTTT RRRREEEELLLLAAAATTTTIIIIOOOONNNNSSSSHHHHIIIIPPPPSSSS The following demonstrates how one might implement "containing" and "using" relationships between objects. ppppaaaacccckkkkaaaaggggeeee BBBBaaaarrrr;;;; 12/Feb/96 perl 5.002 with 3 PERLBOT(1) User Contributed Perl Documentation PERLBOT(1) ssssuuuubbbb nnnneeeewwww {{{{ mmmmyyyy $$$$ttttyyyyppppeeee ==== sssshhhhiiiifffftttt;;;; mmmmyyyy $$$$sssseeeellllffff ==== {{{{}}}};;;; $$$$sssseeeellllffff---->>>>{{{{''''bbbbuuuuzzzz''''}}}} ==== 44442222;;;; bbbblllleeeessssssss $$$$sssseeeellllffff,,,, $$$$ttttyyyyppppeeee;;;; }}}} ppppaaaacccckkkkaaaaggggeeee FFFFoooooooo;;;; ssssuuuubbbb nnnneeeewwww {{{{ mmmmyyyy $$$$ttttyyyyppppeeee ==== sssshhhhiiiifffftttt;;;; mmmmyyyy $$$$sssseeeellllffff ==== {{{{}}}};;;; $$$$sssseeeellllffff---->>>>{{{{''''BBBBaaaarrrr''''}}}} ==== BBBBaaaarrrr---->>>>nnnneeeewwww;;;; $$$$sssseeeellllffff---->>>>{{{{''''bbbbiiiizzzz''''}}}} ==== 11111111;;;; bbbblllleeeessssssss $$$$sssseeeellllffff,,,, $$$$ttttyyyyppppeeee;;;; }}}} ppppaaaacccckkkkaaaaggggeeee mmmmaaaaiiiinnnn;;;; $$$$aaaa ==== FFFFoooooooo---->>>>nnnneeeewwww;;;; pppprrrriiiinnnntttt """"bbbbuuuuzzzz ==== """",,,, $$$$aaaa---->>>>{{{{''''BBBBaaaarrrr''''}}}}---->>>>{{{{''''bbbbuuuuzzzz''''}}}},,,, """"\\\\nnnn"""";;;; pppprrrriiiinnnntttt """"bbbbiiiizzzz ==== """",,,, $$$$aaaa---->>>>{{{{''''bbbbiiiizzzz''''}}}},,,, """"\\\\nnnn"""";;;; OOOOVVVVEEEERRRRRRRRIIIIDDDDIIIINNNNGGGG SSSSUUUUPPPPEEEERRRRCCCCLLLLAAAASSSSSSSS MMMMEEEETTTTHHHHOOOODDDDSSSS The following example demonstrates how to override a superclass method and then call the overridden method. The SSSSUUUUPPPPEEEERRRR pseudo-class allows the programmer to call an overridden superclass method without actually knowing where that method is defined. ppppaaaacccckkkkaaaaggggeeee BBBBuuuuzzzz;;;; ssssuuuubbbb ggggoooooooo {{{{ pppprrrriiiinnnntttt """"hhhheeeerrrreeee''''ssss tttthhhheeee ggggoooooooo\\\\nnnn"""" }}}} ppppaaaacccckkkkaaaaggggeeee BBBBaaaarrrr;;;; @@@@IIIISSSSAAAA ==== qqqqwwww(((( BBBBuuuuzzzz ))));;;; ssssuuuubbbb ggggoooooooogggglllleeee {{{{ pppprrrriiiinnnntttt """"ggggoooooooogggglllleeee hhhheeeerrrreeee\\\\nnnn"""" }}}} ppppaaaacccckkkkaaaaggggeeee BBBBaaaazzzz;;;; ssssuuuubbbb mmmmuuuummmmbbbblllleeee {{{{ pppprrrriiiinnnntttt """"mmmmuuuummmmbbbblllliiiinnnngggg\\\\nnnn"""" }}}} ppppaaaacccckkkkaaaaggggeeee FFFFoooooooo;;;; @@@@IIIISSSSAAAA ==== qqqqwwww(((( BBBBaaaarrrr BBBBaaaazzzz ))));;;; 12/Feb/96 perl 5.002 with 4 PERLBOT(1) User Contributed Perl Documentation PERLBOT(1) ssssuuuubbbb nnnneeeewwww {{{{ mmmmyyyy $$$$ttttyyyyppppeeee ==== sssshhhhiiiifffftttt;;;; bbbblllleeeessssssss [[[[]]]],,,, $$$$ttttyyyyppppeeee;;;; }}}} ssssuuuubbbb ggggrrrrrrrr {{{{ pppprrrriiiinnnntttt """"ggggrrrruuuummmmbbbblllleeee\\\\nnnn"""" }}}} ssssuuuubbbb ggggoooooooo {{{{ mmmmyyyy $$$$sssseeeellllffff ==== sssshhhhiiiifffftttt;;;; $$$$sssseeeellllffff---->>>>SSSSUUUUPPPPEEEERRRR::::::::ggggoooooooo(((())));;;; }}}} ssssuuuubbbb mmmmuuuummmmbbbblllleeee {{{{ mmmmyyyy $$$$sssseeeellllffff ==== sssshhhhiiiifffftttt;;;; $$$$sssseeeellllffff---->>>>SSSSUUUUPPPPEEEERRRR::::::::mmmmuuuummmmbbbblllleeee(((())));;;; }}}} ssssuuuubbbb ggggoooooooogggglllleeee {{{{ mmmmyyyy $$$$sssseeeellllffff ==== sssshhhhiiiifffftttt;;;; $$$$sssseeeellllffff---->>>>SSSSUUUUPPPPEEEERRRR::::::::ggggoooooooogggglllleeee(((())));;;; }}}} ppppaaaacccckkkkaaaaggggeeee mmmmaaaaiiiinnnn;;;; $$$$ffffoooooooo ==== FFFFoooooooo---->>>>nnnneeeewwww;;;; $$$$ffffoooooooo---->>>>mmmmuuuummmmbbbblllleeee;;;; $$$$ffffoooooooo---->>>>ggggrrrrrrrr;;;; $$$$ffffoooooooo---->>>>ggggoooooooo;;;; $$$$ffffoooooooo---->>>>ggggoooooooogggglllleeee;;;; UUUUSSSSIIIINNNNGGGG RRRREEEELLLLAAAATTTTIIIIOOOONNNNSSSSHHHHIIIIPPPP WWWWIIIITTTTHHHH SSSSDDDDBBBBMMMM This example demonstrates an interface for the SDBM class. This creates a "using" relationship between the SDBM class and the new class Mydbm. ppppaaaacccckkkkaaaaggggeeee MMMMyyyyddddbbbbmmmm;;;; rrrreeeeqqqquuuuiiiirrrreeee SSSSDDDDBBBBMMMM____FFFFiiiilllleeee;;;; rrrreeeeqqqquuuuiiiirrrreeee TTTTiiiieeee::::::::HHHHaaaasssshhhh;;;; @@@@IIIISSSSAAAA ==== qqqqwwww(((( TTTTiiiieeee::::::::HHHHaaaasssshhhh ))));;;; 12/Feb/96 perl 5.002 with 5 PERLBOT(1) User Contributed Perl Documentation PERLBOT(1) ssssuuuubbbb TTTTIIIIEEEEHHHHAAAASSSSHHHH {{{{ mmmmyyyy $$$$ttttyyyyppppeeee ==== sssshhhhiiiifffftttt;;;; mmmmyyyy $$$$rrrreeeeffff ==== SSSSDDDDBBBBMMMM____FFFFiiiilllleeee---->>>>nnnneeeewwww((((@@@@____))));;;; bbbblllleeeessssssss {{{{''''ddddbbbbmmmm'''' ====>>>> $$$$rrrreeeeffff}}}},,,, $$$$ttttyyyyppppeeee;;;; }}}} ssssuuuubbbb FFFFEEEETTTTCCCCHHHH {{{{ mmmmyyyy $$$$sssseeeellllffff ==== sssshhhhiiiifffftttt;;;; mmmmyyyy $$$$rrrreeeeffff ==== $$$$sssseeeellllffff---->>>>{{{{''''ddddbbbbmmmm''''}}}};;;; $$$$rrrreeeeffff---->>>>FFFFEEEETTTTCCCCHHHH((((@@@@____))));;;; }}}} ssssuuuubbbb SSSSTTTTOOOORRRREEEE {{{{ mmmmyyyy $$$$sssseeeellllffff ==== sssshhhhiiiifffftttt;;;; iiiiffff ((((ddddeeeeffffiiiinnnneeeedddd $$$$____[[[[0000]]]])))){{{{ mmmmyyyy $$$$rrrreeeeffff ==== $$$$sssseeeellllffff---->>>>{{{{''''ddddbbbbmmmm''''}}}};;;; $$$$rrrreeeeffff---->>>>SSSSTTTTOOOORRRREEEE((((@@@@____))));;;; }}}} eeeellllsssseeee {{{{ ddddiiiieeee """"CCCCaaaannnnnnnnooootttt SSSSTTTTOOOORRRREEEE aaaannnn uuuunnnnddddeeeeffffiiiinnnneeeedddd kkkkeeeeyyyy iiiinnnn MMMMyyyyddddbbbbmmmm\\\\nnnn"""";;;; }}}} }}}} ppppaaaacccckkkkaaaaggggeeee mmmmaaaaiiiinnnn;;;; uuuusssseeee FFFFccccnnnnttttllll qqqqwwww(((( OOOO____RRRRDDDDWWWWRRRR OOOO____CCCCRRRREEEEAAAATTTT ))));;;; ttttiiiieeee %%%%ffffoooooooo,,,, MMMMyyyyddddbbbbmmmm,,,, """"SSSSddddbbbbmmmm"""",,,, OOOO____RRRRDDDDWWWWRRRR||||OOOO____CCCCRRRREEEEAAAATTTT,,,, 0000666644440000;;;; $$$$ffffoooooooo{{{{''''bbbbaaaarrrr''''}}}} ==== 111122223333;;;; pppprrrriiiinnnntttt """"ffffoooooooo----bbbbaaaarrrr ==== $$$$ffffoooooooo{{{{''''bbbbaaaarrrr''''}}}}\\\\nnnn"""";;;; ttttiiiieeee %%%%bbbbaaaarrrr,,,, MMMMyyyyddddbbbbmmmm,,,, """"SSSSddddbbbbmmmm2222"""",,,, OOOO____RRRRDDDDWWWWRRRR||||OOOO____CCCCRRRREEEEAAAATTTT,,,, 0000666644440000;;;; $$$$bbbbaaaarrrr{{{{''''CCCCaaaatttthhhhyyyy''''}}}} ==== 444455556666;;;; pppprrrriiiinnnntttt """"bbbbaaaarrrr----CCCCaaaatttthhhhyyyy ==== $$$$bbbbaaaarrrr{{{{''''CCCCaaaatttthhhhyyyy''''}}}}\\\\nnnn"""";;;; TTTTHHHHIIIINNNNKKKKIIIINNNNGGGG OOOOFFFF CCCCOOOODDDDEEEE RRRREEEEUUUUSSSSEEEE One strength of Object-Oriented languages is the ease with which old code can use new code. The following examples will demonstrate first how one can hinder code reuse and then how one can promote code reuse. This first example illustrates a class which uses a fully- qualified method call to access the "private" method _B_A_Z_(_). The second example will show that it is impossible to override the _B_A_Z_(_) method. ppppaaaacccckkkkaaaaggggeeee FFFFOOOOOOOO;;;; ssssuuuubbbb nnnneeeewwww {{{{ mmmmyyyy $$$$ttttyyyyppppeeee ==== sssshhhhiiiifffftttt;;;; bbbblllleeeessssssss {{{{}}}},,,, $$$$ttttyyyyppppeeee;;;; }}}} ssssuuuubbbb bbbbaaaarrrr {{{{ mmmmyyyy $$$$sssseeeellllffff ==== sssshhhhiiiifffftttt;;;; $$$$sssseeeellllffff---->>>>FFFFOOOOOOOO::::::::pppprrrriiiivvvvaaaatttteeee::::::::BBBBAAAAZZZZ;;;; }}}} 12/Feb/96 perl 5.002 with 6 PERLBOT(1) User Contributed Perl Documentation PERLBOT(1) ppppaaaacccckkkkaaaaggggeeee FFFFOOOOOOOO::::::::pppprrrriiiivvvvaaaatttteeee;;;; ssssuuuubbbb BBBBAAAAZZZZ {{{{ pppprrrriiiinnnntttt """"iiiinnnn BBBBAAAAZZZZ\\\\nnnn"""";;;; }}}} ppppaaaacccckkkkaaaaggggeeee mmmmaaaaiiiinnnn;;;; $$$$aaaa ==== FFFFOOOOOOOO---->>>>nnnneeeewwww;;;; $$$$aaaa---->>>>bbbbaaaarrrr;;;; Now we try to override the _B_A_Z_(_) method. We would like _F_O_O_:_:_b_a_r_(_) to call _G_O_O_P_:_:_B_A_Z_(_), but this cannot happen because _F_O_O_:_:_b_a_r_(_) explicitly calls _F_O_O_:_:_p_r_i_v_a_t_e_:_:_B_A_Z_(_). ppppaaaacccckkkkaaaaggggeeee FFFFOOOOOOOO;;;; ssssuuuubbbb nnnneeeewwww {{{{ mmmmyyyy $$$$ttttyyyyppppeeee ==== sssshhhhiiiifffftttt;;;; bbbblllleeeessssssss {{{{}}}},,,, $$$$ttttyyyyppppeeee;;;; }}}} ssssuuuubbbb bbbbaaaarrrr {{{{ mmmmyyyy $$$$sssseeeellllffff ==== sssshhhhiiiifffftttt;;;; $$$$sssseeeellllffff---->>>>FFFFOOOOOOOO::::::::pppprrrriiiivvvvaaaatttteeee::::::::BBBBAAAAZZZZ;;;; }}}} ppppaaaacccckkkkaaaaggggeeee FFFFOOOOOOOO::::::::pppprrrriiiivvvvaaaatttteeee;;;; ssssuuuubbbb BBBBAAAAZZZZ {{{{ pppprrrriiiinnnntttt """"iiiinnnn BBBBAAAAZZZZ\\\\nnnn"""";;;; }}}} ppppaaaacccckkkkaaaaggggeeee GGGGOOOOOOOOPPPP;;;; @@@@IIIISSSSAAAA ==== qqqqwwww(((( FFFFOOOOOOOO ))));;;; ssssuuuubbbb nnnneeeewwww {{{{ mmmmyyyy $$$$ttttyyyyppppeeee ==== sssshhhhiiiifffftttt;;;; bbbblllleeeessssssss {{{{}}}},,,, $$$$ttttyyyyppppeeee;;;; }}}} ssssuuuubbbb BBBBAAAAZZZZ {{{{ pppprrrriiiinnnntttt """"iiiinnnn GGGGOOOOOOOOPPPP::::::::BBBBAAAAZZZZ\\\\nnnn"""";;;; }}}} ppppaaaacccckkkkaaaaggggeeee mmmmaaaaiiiinnnn;;;; $$$$aaaa ==== GGGGOOOOOOOOPPPP---->>>>nnnneeeewwww;;;; $$$$aaaa---->>>>bbbbaaaarrrr;;;; To create reusable code we must modify class FOO, flattening class FOO::private. The next example shows a reusable class FOO which allows the method _G_O_O_P_:_:_B_A_Z_(_) to be used in place of _F_O_O_:_:_B_A_Z_(_). ppppaaaacccckkkkaaaaggggeeee FFFFOOOOOOOO;;;; 12/Feb/96 perl 5.002 with 7 PERLBOT(1) User Contributed Perl Documentation PERLBOT(1) ssssuuuubbbb nnnneeeewwww {{{{ mmmmyyyy $$$$ttttyyyyppppeeee ==== sssshhhhiiiifffftttt;;;; bbbblllleeeessssssss {{{{}}}},,,, $$$$ttttyyyyppppeeee;;;; }}}} ssssuuuubbbb bbbbaaaarrrr {{{{ mmmmyyyy $$$$sssseeeellllffff ==== sssshhhhiiiifffftttt;;;; $$$$sssseeeellllffff---->>>>BBBBAAAAZZZZ;;;; }}}} ssssuuuubbbb BBBBAAAAZZZZ {{{{ pppprrrriiiinnnntttt """"iiiinnnn BBBBAAAAZZZZ\\\\nnnn"""";;;; }}}} ppppaaaacccckkkkaaaaggggeeee GGGGOOOOOOOOPPPP;;;; @@@@IIIISSSSAAAA ==== qqqqwwww(((( FFFFOOOOOOOO ))));;;; ssssuuuubbbb nnnneeeewwww {{{{ mmmmyyyy $$$$ttttyyyyppppeeee ==== sssshhhhiiiifffftttt;;;; bbbblllleeeessssssss {{{{}}}},,,, $$$$ttttyyyyppppeeee;;;; }}}} ssssuuuubbbb BBBBAAAAZZZZ {{{{ pppprrrriiiinnnntttt """"iiiinnnn GGGGOOOOOOOOPPPP::::::::BBBBAAAAZZZZ\\\\nnnn"""";;;; }}}} ppppaaaacccckkkkaaaaggggeeee mmmmaaaaiiiinnnn;;;; $$$$aaaa ==== GGGGOOOOOOOOPPPP---->>>>nnnneeeewwww;;;; $$$$aaaa---->>>>bbbbaaaarrrr;;;; CCCCLLLLAAAASSSSSSSS CCCCOOOONNNNTTTTEEEEXXXXTTTT AAAANNNNDDDD TTTTHHHHEEEE OOOOBBBBJJJJEEEECCCCTTTT Use the object to solve package and class context problems. Everything a method needs should be available via the object or should be passed as a parameter to the method. A class will sometimes have static or global data to be used by the methods. A subclass may want to override that data and replace it with new data. When this happens the superclass may not know how to find the new copy of the data. This problem can be solved by using the object to define the context of the method. Let the method look in the object for a reference to the data. The alternative is to force the method to go hunting for the data ("Is it in my class, or in a subclass? Which subclass?"), and this can be inconvenient and will lead to hackery. It is better to just let the object tell the method where that data is located. ppppaaaacccckkkkaaaaggggeeee BBBBaaaarrrr;;;; %%%%ffffiiiizzzzzzzzlllleeee ==== (((( ''''PPPPaaaasssssssswwwwoooorrrrdddd'''' ====>>>> ''''XXXXYYYYZZZZZZZZYYYY'''' ))));;;; 12/Feb/96 perl 5.002 with 8 PERLBOT(1) User Contributed Perl Documentation PERLBOT(1) ssssuuuubbbb nnnneeeewwww {{{{ mmmmyyyy $$$$ttttyyyyppppeeee ==== sssshhhhiiiifffftttt;;;; mmmmyyyy $$$$sssseeeellllffff ==== {{{{}}}};;;; $$$$sssseeeellllffff---->>>>{{{{''''ffffiiiizzzzzzzzlllleeee''''}}}} ==== \\\\%%%%ffffiiiizzzzzzzzlllleeee;;;; bbbblllleeeessssssss $$$$sssseeeellllffff,,,, $$$$ttttyyyyppppeeee;;;; }}}} ssssuuuubbbb eeeennnntttteeeerrrr {{{{ mmmmyyyy $$$$sssseeeellllffff ==== sssshhhhiiiifffftttt;;;; #### DDDDoooonnnn''''tttt ttttrrrryyyy ttttoooo gggguuuueeeessssssss iiiiffff wwwweeee sssshhhhoooouuuulllldddd uuuusssseeee %%%%BBBBaaaarrrr::::::::ffffiiiizzzzzzzzlllleeee #### oooorrrr %%%%FFFFoooooooo::::::::ffffiiiizzzzzzzzlllleeee.... TTTThhhheeee oooobbbbjjjjeeeecccctttt aaaallllrrrreeeeaaaaddddyyyy kkkknnnnoooowwwwssss wwwwhhhhiiiicccchhhh #### wwwweeee sssshhhhoooouuuulllldddd uuuusssseeee,,,, ssssoooo jjjjuuuusssstttt aaaasssskkkk iiiitttt.... #### mmmmyyyy $$$$ffffiiiizzzzzzzzlllleeee ==== $$$$sssseeeellllffff---->>>>{{{{''''ffffiiiizzzzzzzzlllleeee''''}}}};;;; pppprrrriiiinnnntttt """"TTTThhhheeee wwwwoooorrrrdddd iiiissss """",,,, $$$$ffffiiiizzzzzzzzlllleeee---->>>>{{{{''''PPPPaaaasssssssswwwwoooorrrrdddd''''}}}},,,, """"\\\\nnnn"""";;;; }}}} ppppaaaacccckkkkaaaaggggeeee FFFFoooooooo;;;; @@@@IIIISSSSAAAA ==== qqqqwwww(((( BBBBaaaarrrr ))));;;; %%%%ffffiiiizzzzzzzzlllleeee ==== (((( ''''PPPPaaaasssssssswwwwoooorrrrdddd'''' ====>>>> ''''RRRRuuuummmmpppplllleeee'''' ))));;;; ssssuuuubbbb nnnneeeewwww {{{{ mmmmyyyy $$$$ttttyyyyppppeeee ==== sssshhhhiiiifffftttt;;;; mmmmyyyy $$$$sssseeeellllffff ==== BBBBaaaarrrr---->>>>nnnneeeewwww;;;; $$$$sssseeeellllffff---->>>>{{{{''''ffffiiiizzzzzzzzlllleeee''''}}}} ==== \\\\%%%%ffffiiiizzzzzzzzlllleeee;;;; bbbblllleeeessssssss $$$$sssseeeellllffff,,,, $$$$ttttyyyyppppeeee;;;; }}}} ppppaaaacccckkkkaaaaggggeeee mmmmaaaaiiiinnnn;;;; $$$$aaaa ==== BBBBaaaarrrr---->>>>nnnneeeewwww;;;; $$$$bbbb ==== FFFFoooooooo---->>>>nnnneeeewwww;;;; $$$$aaaa---->>>>eeeennnntttteeeerrrr;;;; $$$$bbbb---->>>>eeeennnntttteeeerrrr;;;; IIIINNNNHHHHEEEERRRRIIIITTTTIIIINNNNGGGG AAAA CCCCOOOONNNNSSSSTTTTRRRRUUUUCCCCTTTTOOOORRRR An inheritable constructor should use the second form of _b_l_e_s_s_(_) which allows blessing directly into a specified class. Notice in this example that the object will be a BAR not a FOO, even though the constructor is in class FOO. ppppaaaacccckkkkaaaaggggeeee FFFFOOOOOOOO;;;; ssssuuuubbbb nnnneeeewwww {{{{ mmmmyyyy $$$$ttttyyyyppppeeee ==== sssshhhhiiiifffftttt;;;; mmmmyyyy $$$$sssseeeellllffff ==== {{{{}}}};;;; bbbblllleeeessssssss $$$$sssseeeellllffff,,,, $$$$ttttyyyyppppeeee;;;; }}}} 12/Feb/96 perl 5.002 with 9 PERLBOT(1) User Contributed Perl Documentation PERLBOT(1) ssssuuuubbbb bbbbaaaazzzz {{{{ pppprrrriiiinnnntttt """"iiiinnnn FFFFOOOOOOOO::::::::bbbbaaaazzzz(((())))\\\\nnnn"""";;;; }}}} ppppaaaacccckkkkaaaaggggeeee BBBBAAAARRRR;;;; @@@@IIIISSSSAAAA ==== qqqqwwww((((FFFFOOOOOOOO))));;;; ssssuuuubbbb bbbbaaaazzzz {{{{ pppprrrriiiinnnntttt """"iiiinnnn BBBBAAAARRRR::::::::bbbbaaaazzzz(((())))\\\\nnnn"""";;;; }}}} ppppaaaacccckkkkaaaaggggeeee mmmmaaaaiiiinnnn;;;; $$$$aaaa ==== BBBBAAAARRRR---->>>>nnnneeeewwww;;;; $$$$aaaa---->>>>bbbbaaaazzzz;;;; DDDDEEEELLLLEEEEGGGGAAAATTTTIIIIOOOONNNN Some classes, such as SDBM_File, cannot be effectively subclassed because they create foreign objects. Such a class can be extended with some sort of aggregation technique such as the "using" relationship mentioned earlier or by delegation. The following example demonstrates delegation using an _A_U_T_O_L_O_A_D_(_) function to perform message-forwarding. This will allow the Mydbm object to behave exactly like an SDBM_File object. The Mydbm class could now extend the behavior by adding custom _F_E_T_C_H_(_) and _S_T_O_R_E_(_) methods, if this is desired. ppppaaaacccckkkkaaaaggggeeee MMMMyyyyddddbbbbmmmm;;;; rrrreeeeqqqquuuuiiiirrrreeee SSSSDDDDBBBBMMMM____FFFFiiiilllleeee;;;; rrrreeeeqqqquuuuiiiirrrreeee TTTTiiiieeee::::::::HHHHaaaasssshhhh;;;; @@@@IIIISSSSAAAA ==== qqqqwwww((((TTTTiiiieeee::::::::HHHHaaaasssshhhh))));;;; ssssuuuubbbb TTTTIIIIEEEEHHHHAAAASSSSHHHH {{{{ mmmmyyyy $$$$ttttyyyyppppeeee ==== sssshhhhiiiifffftttt;;;; mmmmyyyy $$$$rrrreeeeffff ==== SSSSDDDDBBBBMMMM____FFFFiiiilllleeee---->>>>nnnneeeewwww((((@@@@____))));;;; bbbblllleeeessssssss {{{{''''ddddeeeelllleeeeggggaaaatttteeee'''' ====>>>> $$$$rrrreeeeffff}}}};;;; }}}} ssssuuuubbbb AAAAUUUUTTTTOOOOLLLLOOOOAAAADDDD {{{{ mmmmyyyy $$$$sssseeeellllffff ==== sssshhhhiiiifffftttt;;;; #### TTTThhhheeee PPPPeeeerrrrllll iiiinnnntttteeeerrrrpppprrrreeeetttteeeerrrr ppppllllaaaacccceeeessss tttthhhheeee nnnnaaaammmmeeee ooooffff tttthhhheeee #### mmmmeeeessssssssaaaaggggeeee iiiinnnn aaaa vvvvaaaarrrriiiiaaaabbbblllleeee ccccaaaalllllllleeeedddd $$$$AAAAUUUUTTTTOOOOLLLLOOOOAAAADDDD.... #### DDDDEEEESSSSTTTTRRRROOOOYYYY mmmmeeeessssssssaaaaggggeeeessss sssshhhhoooouuuulllldddd nnnneeeevvvveeeerrrr bbbbeeee pppprrrrooooppppaaaaggggaaaatttteeeedddd.... rrrreeeettttuuuurrrrnnnn iiiiffff $$$$AAAAUUUUTTTTOOOOLLLLOOOOAAAADDDD ====~~~~ ////::::::::DDDDEEEESSSSTTTTRRRROOOOYYYY$$$$////;;;; #### RRRReeeemmmmoooovvvveeee tttthhhheeee ppppaaaacccckkkkaaaaggggeeee nnnnaaaammmmeeee.... $$$$AAAAUUUUTTTTOOOOLLLLOOOOAAAADDDD ====~~~~ ssss////^^^^MMMMyyyyddddbbbbmmmm::::::::////////;;;; 12/Feb/96 perl 5.002 with 10 PERLBOT(1) User Contributed Perl Documentation PERLBOT(1) #### PPPPaaaassssssss tttthhhheeee mmmmeeeessssssssaaaaggggeeee ttttoooo tttthhhheeee ddddeeeelllleeeeggggaaaatttteeee.... $$$$sssseeeellllffff---->>>>{{{{''''ddddeeeelllleeeeggggaaaatttteeee''''}}}}---->>>>$$$$AAAAUUUUTTTTOOOOLLLLOOOOAAAADDDD((((@@@@____))));;;; }}}} ppppaaaacccckkkkaaaaggggeeee mmmmaaaaiiiinnnn;;;; uuuusssseeee FFFFccccnnnnttttllll qqqqwwww(((( OOOO____RRRRDDDDWWWWRRRR OOOO____CCCCRRRREEEEAAAATTTT ))));;;; ttttiiiieeee %%%%ffffoooooooo,,,, MMMMyyyyddddbbbbmmmm,,,, """"aaaaddddbbbbmmmm"""",,,, OOOO____RRRRDDDDWWWWRRRR||||OOOO____CCCCRRRREEEEAAAATTTT,,,, 0000666644440000;;;; $$$$ffffoooooooo{{{{''''bbbbaaaarrrr''''}}}} ==== 111122223333;;;; pppprrrriiiinnnntttt """"ffffoooooooo----bbbbaaaarrrr ==== $$$$ffffoooooooo{{{{''''bbbbaaaarrrr''''}}}}\\\\nnnn"""";;;; 12/Feb/96 perl 5.002 with 11