Re: I can not compile code from <<modern c++ design>>

[Matthias Oltmanns <Mathias dot Oltmanns dot Oltmanns at sysde dot eads dot net>]

Am Mon, 2003-04-07 um 15.54 schrieb Di Yang:
> At 13:02 4/7/2003 +0200, Matthias Oltmanns wrote:
> >Am Mon, 2003-04-07 um 11.24 schrieb Di Yang:
> > > Hi,
> > >
> > > public:
> > > enum { exists = sizeof(Test(MakeT())) == sizeof(Small) };
> >
> >Hi,
> >
> >this seems to be not valid C++ code because the calls to the
> >static members 'Test' and 'MakeT' are not const expressions at compile
> >time.
> >For assignments to enum literals there are only integer expressions
> >allowed which are evaluated at compile time.
> >
> >cu
> >Matthias
> 
> Thank you, I changed the code as following and passed GCC 3.2:  -Di
> 
>          template <class T, class U>
>          struct ConversionHelper
>          {
>              typedef char Small;
>              struct Big { char dummy[2]; };
>              static Big   Test(...);
>              static Small Test(U);
>              static T MakeT();
>          };
> 
>      template <class T, class U>
>      struct Conversion
>      {
>          typedef ConversionHelper<T, U> H;
>          enum { exists = sizeof(typename H::Small) == 
> sizeof(H::Test(H::MakeT())) };
> 
>      };
> ......
> 
> 
> C:\ttt>g++ -o template template.cpp
> template.cpp: In instantiation of `Conversion<double, int>':
> template.cpp:40:   instantiated from here
> template.cpp:40: warning: passing `double' for argument 1 of `static char
>     ConversionHelper<T, U>::Test(U) [with T = double, U = int]'
> 
> C:\ttt>template
> 1 0
> C:\ttt>

Hi,

i'am confused. Your changes shows that I'm wrong. But i' cant see why.
You could also change your first version making all members public:

class Conversion
{
public:
  typedef char Small;
  class Big { char dummy[2]; };
  
  static Small Test(U);
  static Big Test(...);
  static T MakeT();
  
  enum { exists = sizeof(Test(MakeT())) == sizeof(Small) };
};

which also works.

So, if the initialization of the enum is legal it should be legal in
your previous version having only the enum public.

cu
Matthias