C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  structural inheritance bad?          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message davidrubin@warpmail.net Guest Posted: Fri Jan 28, 2005 5:09 am    Post subject: structural inheritance bad? Structural inheritance (inheriting implementation) is equivalent to composition in that a particular method must either call 'Base::foo' or invoke 'base.foo'. Apparantly, The Literature tells us to prefer composition over inheritance. Can anyone provide some reasons why this is the case (based on "real-world" experience)? For example, is structural inheritance more difficult to maintain? More difficult to test? Have a larger impact on compile time? Not lend itself well to re-use? Thanks. /david Back to top Alf P. Steinbach Guest Posted: Fri Jan 28, 2005 5:21 am    Post subject: Re: structural inheritance bad? * [email]davidrubin (AT) warpmail (DOT) net[/email]: Quote: [muddling oop thoughts] OT. -- A: Because it messes up the order in which people normally read text. Q: Why is it such a bad thing? A: Top-posting. Q: What is the most annoying thing on usenet and in e-mail? Back to top davidrubin@warpmail.net Guest Posted: Fri Jan 28, 2005 5:35 am    Post subject: Re: structural inheritance bad? I see, clc++ is like physics: it only answers "how", never "why". Let me try again: what are the implications of using structural inheritance versus composition in a large C++ project? Back to top Ioannis Vranos Guest Posted: Fri Jan 28, 2005 5:40 am    Post subject: Re: structural inheritance bad? [email]davidrubin (AT) warpmail (DOT) net[/email] wrote: Quote: I see, clc++ is like physics: it only answers "how", never "why". Let me try again: what are the implications of using structural inheritance versus composition in a large C++ project? I think you are looking for some "silver bullet". Check this article regarding "silver bullets": http://www.itworld.com/AppDev/710/lw-02-stroustrup/page_1.html -- Ioannis Vranos http://www23.brinkster.com/noicys Back to top Adios Guest Posted: Fri Jan 28, 2005 10:59 am    Post subject: Re: structural inheritance bad? Ioannis Vranos wrote: Quote: davidrubin (AT) warpmail (DOT) net wrote: I see, clc++ is like physics: it only answers "how", never "why". Let me try again: what are the implications of using structural inheritance versus composition in a large C++ project? I think you are looking for some "silver bullet". Check this article regarding "silver bullets": http://www.itworld.com/AppDev/710/lw-02-stroustrup/page_1.html [Q. what would you lie to see added to the standard?] Bjarne Stroustrup: GUI: It would be nice to have a standard GUI framework, but I don't see how that could be politically feasible. Platform-independent system facilities: I'd like to see the Standard Library provide a broader range of standard interfaces to common system resources (where available), such as directories and sockets. Hey no fair Stroustrup! What would be left for old Dead Dick Vic to moan about? Back to top davidrubin@warpmail.net Guest Posted: Fri Jan 28, 2005 1:13 pm    Post subject: Re: structural inheritance bad? This article does not seem to address my question in the slightest, but thanks. Back to top mwigdahl Guest Posted: Fri Jan 28, 2005 7:38 pm    Post subject: Re: structural inheritance bad? Structural inheritance is fine if your design respects the LSP (Liskov Substitution Principle) -- basically, that all operations that operate on base-class objects can have derived-class objects substituted and operate correctly. Composition is safer in that it doesn't inherently advertise LSP conformance, although you tend to type more. :) I have found that the problem is that although it is fairly easy to design hierarchies respecting the LSP at the start of a large project, modifications made in midstream have a good chance of causing LSP violations if your relationships really shouldn't have been "IS-A" to begin with, leading to either kludges or extensive refactoring. YMMV. More info on the LSP: http://www.objectmentor.com/resources/articles/lsp.pdf Matt Back to top Jesper Madsen Guest Posted: Fri Jan 28, 2005 8:43 pm    Post subject: Re: structural inheritance bad? <davidrubin (AT) warpmail (DOT) net> wrote Quote: Structural inheritance (inheriting implementation) is equivalent to composition in that a particular method must either call 'Base::foo' or invoke 'base.foo'. Apparantly, The Literature tells us to prefer composition over inheritance. Can anyone provide some reasons why this is the case (based on "real-world" experience)? For example, is structural inheritance more difficult to maintain? More difficult to test? Have a larger impact on compile time? Not lend itself well to re-use? Thanks. /david If you inherit to add functionality, the child needs to #include the parent in the header. If you add functionality by aggregating, you can choose to just add a pointer to the class. The benefit of this would be, that you have isolated the knowledge to the cpp files that uses the header. It is worth a lot, when you are working on large projects. Back to top Andrey Tarasevich Guest Posted: Fri Jan 28, 2005 9:49 pm    Post subject: Re: structural inheritance bad? mwigdahl wrote: Quote: Structural inheritance is fine if your design respects the LSP (Liskov Substitution Principle) -- basically, that all operations that operate on base-class objects can have derived-class objects substituted and operate correctly. Composition is safer in that it doesn't inherently advertise LSP conformance, although you tend to type more. Huh? OP is talking about inheritance that does not implement the IS-A relationship. There's abosolutely no requirement for this type of inheritance to satisfy LSP. LSP is simply not applicable to this type of inheritance. -- Best regards, Andrey Tarasevich Back to top Andrey Tarasevich Guest Posted: Fri Jan 28, 2005 10:02 pm    Post subject: Re: structural inheritance bad? [email]davidrubin (AT) warpmail (DOT) net[/email] wrote: Quote: Structural inheritance (inheriting implementation) is equivalent to composition in that a particular method must either call 'Base::foo' or invoke 'base.foo'. Apparantly, The Literature tells us to prefer composition over inheritance. Can anyone provide some reasons why this is the case (based on "real-world" experience)? For example, is structural inheritance more difficult to maintain? More difficult to test? Have a larger impact on compile time? Not lend itself well to re-use? There are absolutely no serious problems with this type of inheritance as long as it is implemented through private inheritance. Although it is worth pointing out that aggregation (as opposed to inheritance) is in general case more flexible. It is possible to aggregate several instances of the same class, if necessary. You can also give a meaningful name to the aggregated object, which makes the program more readable. Neither is possible is case of inheritance. Whether all this really makes a noticeable difference depends on the concrete situation. However, sometimes in C++ language one has to employ _public_ structural (i.e. non-IS-A) inheritance in order to use certain convenient and well-established implementational techniques. This is often frowned upon because the popular belief is that public inheritance in C++ shall only be used to implement IS-A relationships (LSP, etc.). I personally don't agree with this view of public inheritance in C++ (or, more precisely, I consider it to be unnecessarily extreme), but don't be surprised if you meet someone who does. -- Best regards, Andrey Tarasevich Back to top mwigdahl Guest Posted: Fri Jan 28, 2005 10:51 pm    Post subject: Re: structural inheritance bad? Yes, I reread (slower, this time) and you are right. Sorry! 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