Re: [boost] Re: compile-time variables

From: Peder Holt (peder.holt_at_[hidden])
Date: 2004-09-03 01:29:47

On Tue, 24 Aug 2004 22:10:22 +0200, Daniel Wallin
<dalwan01_at_[hidden]> wrote:
> Goran Mitrovic wrote:
> > I've got a question.
> >
> > Is a compile-time variables concept known?
> 
> Yes, but AFAIK it can't be done in standard C++. I guess you mean
> something along the lines of this hack, which takes advantage of a
> common compiler bug which will incorrectly find the check() and value()
> overloads.. Works on gcc and maybe something else, but it's not
> portable.
> 
>    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 N>
>    struct set_variable
>    {
>        typedef counter<10> start_type;
> 
>        enum
>        {
>            current = sizeof(check((T*)0, (start_type*)0)) - 1,
>            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, 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";
>    }
> 
> --
> Daniel Wallin
> 

Just a suggestion: 
Making set_variable a parameter to the check and value functions
should make them available for standard compliant compilers as well,
if I have interpreted the standards correctly.

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

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

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

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

This compiles fine for VC 6.5 (so does the original example, so this
is no proof of compliancy)
> 
> 
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