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From: Daniel.Miller@tellabs.com ("Dan'l Miller")
Newsgroups: comp.std.c++
Subject: Re: automatic downcast support through function parameters
Date: Thu, 2 Jan 2003 06:39:24 +0000 (UTC)
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danielgutson@hotmail.com wrote:
> People: I'd like to ask about the feasibility of this:
> automatic downcast during function calls.
> let's have:
>  struct Base{};
>  struct Der : Base{};
> 
>  void g(Base* b);
>  void g(Der* d);
> 
>  void f(upcasted Base* b)    // see below
>  {
>    g(b);
>  }
> 
>  void main(void)
>  {
>     Der d;
>     f(&d);
>  }
> 
>  - The idea here is that f ignores the function that will be finally
>  invoked, but knows that the pointer was upcasted ('upcasted'
> keyword).
>  Then, the function accepting the most derived class (closest to the
>  original type) will be invoked, in this case: g(Der*).
> 
> The 'spirit' of this is to recover specificity: in the process flow:
>   main -> f -> g
> specificity(main) = specificity(g) > specificity(f)
> and this would be an automatic way of recoviring specificity at g,
> without loosing generality at f.
> 
>  What you think?

   I think that this works well at compile-time only 1) if one has a single 
compilation unit or 2) if one has global analysis among separate compilation 
units.  In practice "compilation unit" normally corresponds to a .c/.cpp/.cxx/.C 
file which begets a .o/.obj file.

   Because C's (and thus C++'s) compilation model is predicated on separate 
compilation, this proposed feature is in turn predicated on the hypothetical 
removal of separate compilation from C++/C.  Such removal of the 
separate-compilation axiom appears quite unlikely to me in the foreseeable future.

   I think that human-authored code can be written to 
overtly/manually/nonautomatically accomplish this via RTTI at run-time.

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