Re: [boost] Re: Re: An alternative typeof macro

From: Peder Holt (peder.holt_at_[hidden])
Date: 2004-09-02 01:38:07

On Wed, 1 Sep 2004 14:26:30 -0400, Arkadiy Vertleyb
<vertleyb_at_[hidden]> wrote:
> "Peder Holt" <peder.holt_at_[hidden]> wrote
> 
> > As to the implementation of my code,
> > I got the inspiration to my implementation from Daniel Wallins post
> > some time back on compile time constants:
> > http://lists.boost.org/MailArchives/boost/msg69842.php
> 
> I compiled this code in VC7.1, and it did compile although with warnings.
> 
> But then I modified main to look like this:
> 
> int main()
> {
> SET(X, 10);
> std::cout << CURRENT(X) << "\n";
> std::cout << VALUE(X) << "\n";
> SET(X, 15);
> std::cout << CURRENT(X) << "\n";
> std::cout << VALUE(X) << "\n";
> SET(X, 20);
> std::cout << CURRENT(X) << "\n";
> std::cout << VALUE(X) << "\n";
> SET(X, 15);
> std::cout << CURRENT(X) << "\n";
> std::cout << VALUE(X) << "\n";
> }
> 
> the output was somewhat different than one would expect:
> 
> 1
> 10
> 2
> 15
> 3
> 20
> 3    -- wrong
> 20  -- wrong
> 
> How do you handle this in your typeof implementation?  Do you reset it
> somehow between typeofs?  Or do you start from the next available number?

That is because the class set_variable<X,15> has already been
instantiated. When you instantiate it again, you get the old
implementation of set_variable. To overcome this problem, you need to
add an additional template parameter to the set_variable class:

 template<int N> struct counter : counter<N - 1> {};
  template<> struct counter<0> {};

  template<int N>
  struct size_type
  {
      typedef char(&type)[N + 1];
  };

  size_type<0>::type check(...);

  template<class T, int Current, int N>
  struct set_variable
  {
      typedef counter<10> start_type;

      enum
      {
          current = Current,
          next = current + 1
      };

      friend typename size_type<next>::type check(T*, counter<next>*) {}
      friend typename size_type<N>::type value(T*, counter<current>*) {}
  };

  #define CURRENT(T) (sizeof(check((T*)0, (counter<10>*)0)) - 1)
  #define VALUE(T) (sizeof(value((T*)0, (counter<10>*)0)) - 1)
  #define SET(T, N) set_variable<T,CURRENT(T), N>()

  #include <iostream>

  struct X {};

  int main()
  {
      SET(X, 10);
      std::cout << CURRENT(X) << "\n";
      std::cout << VALUE(X) << "\n";

      SET(X, 15);
      std::cout << CURRENT(X) << "\n";
      std::cout << VALUE(X) << "\n";

      SET(X, 20);
      std::cout << CURRENT(X) << "\n";
      std::cout << VALUE(X) << "\n";

      SET(X, 15);
      std::cout << CURRENT(X) << "\n";
      std::cout << VALUE(X) << "\n";
  }

voila.

--
Peder Holt
> 
> Regards,
> Arkadiy
> 
> 
> 
> 
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