Proposal: Static Invariant (C++ concept in D)

[davidl]

在 Mon, 17 Sep 2007 20:55:12 +0800，Bill Baxter  
<dnewsgroup at billbaxter.com> 写道:

> davidl wrote:
>> I've read a little bit stuff of C++ concept, and find it has some  
>> similarity with
>> D's runtime invariant in some abstraction. In some extent , we can  
>> imagine C++
>> concept is kind of compile time invariant.
>>  So the following:
>>  class TemplateClass(T)
>> {
>>   static invariant()   // reuse keywords, nothing more get introduced
>>   {
>>     // prerequisite checking, does T have foo , does T have bar
>>     Base_Concept_check();  // these emulate the inheritance of concept
>>     Derived_Concept_check();
>>   }
>> }
>
> Can't you just do the same thing now, with slightly different syntax?
>
> For instance Base_Concept_check could be a template mixin with nothing  
> but static ifs and other compile-time checks.  Then you just have to say  
> "mixin Base_Concept_check;" instead of calling it like a function, but  
> same basic thing, no?
>
> --bb

it's a little bit different. Because the check means the prerequisite of  
the template,
This is for the whole template scope. not till the last moment of making a  
call. So
a mixin would trigger the "static invariant".

And "static invariant" itself is for prerequisite checking , so it's an  
enforcement for
template design. The template inner scope can only use the stuffs which  
are checked in
the "static invariant", I mean any stuff or members related to type T.  
Also IDE could
provide nice code completion for template writers.

I think it's for elegance. With some prerequisite checking the template  
system would be
nicer.

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