From 2179972753443407093 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: 109fba,109af18a94b8a35c X-Google-Attributes: gid109fba,public X-Google-Thread: f78e5,e5cae9ffbf1dd57e X-Google-Attributes: gidf78e5,public X-Google-ArrivalTime: 1992-10-05 13:59:46 PST Xref: sparky comp.lang.c++:14547 comp.std.c++:1316 Path: sparky!uunet!snorkelwacker.mit.edu!ai-lab!life.ai.mit.edu!tmb From: tmb@arolla.idiap.ch (Thomas M. Breuel) Newsgroups: comp.lang.c++,comp.std.c++ Subject: Re: The concept of templates considered ill designed Message-ID: Date: 6 Oct 92 00:01:13 GMT References: <23801@alice.att.com> <0enxJ1W00awTMNepU8@andrew.cmu.edu> <1992Oct5.160022.29224@genghis.borland.com> Sender: news@ai.mit.edu Reply-To: tmb@idiap.ch Followup-To: comp.lang.c++ Organization: IDIAP (Institut Dalle Molle d'Intelligence Artificielle Perceptive) Lines: 45 In-reply-to: pete@genghis.borland.com's message of 5 Oct 92 16:00:22 GMT In article <1992Oct5.160022.29224@genghis.borland.com> pete@genghis.borland.com (Pete Becker) writes: [discussion of how to go about adding various forms of type constraints to template arguments deleted] But this has very little to do with templates. Constrained parameters and no further knowledge of the actual type is equivalent, as far as I can see, to simply carrying around a pointer to the actual class, and dealing with it only through the known interface. You don't need templates to implement this, and I don't see that there's anything gained by using the template mechanism to do it. That's not true. Unlike when you rely on inheritance, you can use a template with any type that has the needed operations defined on it. The type doesn't have to inherit from some common base type. The type doesn't even have to be a class/struct. For example, it is quite frequent that people write numerical template code that works with an object that is vaguely number like. You'd like to be able to use it with the built-in numerical types, as well as any user defined types, as well as any types from a commercial library. None of those types will inherit from one another, or from a common "Number" type, nor is it entirely trivial to make this happen. Another important reason is efficiency. Templates will generate code that is specialized for the exact types that I'm using. A third reason is that for templates, unlike for "dealing with classes through a known interface", I can express type relationships between arguments, and between arguments and return types. template A f(A x,A y) { ... } is a very different function from A g(A x,A y) { ... } even if all the objects you hand to "f" are subtypes of "A". Allowing constraints on template arguments has proven to be very useful in other languages. I think such a feature would be very important in C++ as well. I would like to see both inheritance based constraints and signature based constraints. Thomas.