From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me X-Spam-Level: X-Spam-Status: No, score=-0.9 required=5.0 tests=BAYES_00,FORGED_GMAIL_RCVD, FREEMAIL_FROM autolearn=no autolearn_force=no version=3.4.4 X-Google-Thread: 103376,a84eaf8fb2470909 X-Google-Attributes: gid103376,public X-Google-Language: ENGLISH,ASCII-7-bit Path: g2news2.google.com!postnews.google.com!n67g2000cwd.googlegroups.com!not-for-mail From: "markww" Newsgroups: comp.lang.ada Subject: Re: Ada generics Date: 21 Dec 2006 10:22:16 -0800 Organization: http://groups.google.com Message-ID: <1166725336.042400.61300@n67g2000cwd.googlegroups.com> References: <1166710494.869393.108730@a3g2000cwd.googlegroups.com> <1166720103.733870.120450@n67g2000cwd.googlegroups.com> NNTP-Posting-Host: 66.65.64.109 Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" X-Trace: posting.google.com 1166725342 14250 127.0.0.1 (21 Dec 2006 18:22:22 GMT) X-Complaints-To: groups-abuse@google.com NNTP-Posting-Date: Thu, 21 Dec 2006 18:22:22 +0000 (UTC) In-Reply-To: <1166720103.733870.120450@n67g2000cwd.googlegroups.com> User-Agent: G2/1.0 X-HTTP-UserAgent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727),gzip(gfe),gzip(gfe) Complaints-To: groups-abuse@google.com Injection-Info: n67g2000cwd.googlegroups.com; posting-host=66.65.64.109; posting-account=cNKOMg0AAADT2ug8oGSYYXo8bsDvrHzw Xref: g2news2.google.com comp.lang.ada:7975 Date: 2006-12-21T10:22:16-08:00 List-Id: Hyman Rosen wrote: > markww wrote: > > I'm trying to compare generics in Ada vs C++ templates > > There are fundamental differences between the two languages in how > generics work that relate back to how types themselves work. > > In C++, a type is a completely static thing. You can read a type > declaration in the code and know everything there is to know about it. > In Ada, types are much more dynamic - they can include information > known only at run type, such as array sizes and discriminant values. (I > don't know Ada, so pardon me if I'm wrong in the Ada details.) > Accordingly, C++ types don't have a lifetime as such, existing for the > life of the program, but Ada types have the same lifetime as other > objects in the scope in which they are declared. Among other things, > this means that C++ types can be assigned unique names at compile time > while Ada types cannot. > > Now, imagine you have a generic which wants to take a type parameter. > > In C++, because types are fixed and uniquely named, the generic itself > can be instantiated at compile time. The compiler has all the > information it needs. Furthermore, the instantiated generic can also be > uniquely named. This becomes the paradigm of generic instantiation in > C++ - to do it all in the compiler. Thus, generics may take non-type > parameters as well, but they must all be constant (or refer to the > address of a static object). As an additional consequence, C++ is able > to identify instantiations with each other based on identity of the > template parameters so that, for example, the type std::vector in > one file is exactly the same type as std::vector in a second file. > It also makes automatic instantiation of function templates easier, > since again the full set of needed instantiations is known at compile > time, and instantiations with the same set of parameters are identified > with each other. For example, in a function template containing a > static variable, there is one such variable generated for each > different set of instantiation parameters. > > In Ada things are different. Because types have lifetimes and contain > runtime information, generic instantiations must also. Furthermore, in > Ada even functions and procedures have lifetimes, because they nest and > have access to outer variables. Now it is not possible to know at > compile time how many generics need to be instantiated, and you cannot > identify a generic instantiation simply by looking at the generic and > its parameters. Each instantiation is separate, and that makes > automatic instantiation of generic functions problematic, because it > may be difficult to know when to do a new instantiation and when to > reuse an existing one. On the other hand, since you are already so > dynamic, there doesn't need to be any restriction on non-type > parameters, and so in Ada you can instantiate generics on > locally-scoped functions and objects. > > Because(?) Ada requires explicit instantiations, it has no notion of > generic specializations. This means that in Ada you cannot do > metaprogramming the way you can in C++. In Ada a generic has one and > only one implementation, whereas in C++ a template can be specialized > for different sets of template parameters. On the other hand, Ada is > much more friendly to generic sharing, that is, having a single or a > few compiled functions that can act at runtime as particular > instantiated functions. Thanks for your detailed responses, I appreciate it, Mark