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Newsgroups: comp.lang.ada
Subject: Re: prohibit certain generic instantiations in Ada 2005
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From: Stephen Leake <stephen_leake@acm.org>
Message-ID: <uzmkxrjol.fsf@acm.org>
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Date: Sat, 11 Feb 2006 10:36:58 -0500
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Date: 2006-02-11T10:36:58-05:00
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matteo.bordin@gmail.com writes:

> I will provide a proper example in C++. With this code I can prohibit
> instantiation of template My_Class with T=int.
>
> class Checker{
> public:
>   static void check(float){};
> private:
>   static void check(int){};
> };
>
> template <class T>
> class My_Class{
> public:
> My_Class(){
>   T t;
>   Checker::check(t);
>
> }
> };
>
> int main(){
>
>   My_Class<int> mc = My_Class<int>(); //COMPILE TIME ERROR!!!!
>   //The error comes from the fact that Checker::check(int) is private
>   My_Class<float> mc2 = My_Class<float>(); //OK!!!
> };
>
> When I said that the compilation model for template of Ada is similar
> to C++, I meant in regards to compile-time code generation (opposed to
> the model of Java generics). In both languages (Ada and C++), templates
> can be used as a generative environment.

Ok, you _can_ do this in C++. But they question remains; _why_ do you
want to do this? What is it that Checker does that is appropriate for
all types, except 'int'?

> 
>  just want to know f here is a way to reproduce the C++ code I
> provided in Ada (2005).

You can write code in the execution part of a package body (after the
'begin') that does some checking, and raises an exception. Most
compilers will warn at compile time that a run-time construct will
raise an exception.

-- 
-- Stephe