[antlr-interest] Parsing problem in C++

[Arnulf Heller]

hello david,

I think this is the "null value" of class E.
I recently read about that in the template book of Josuttis/Vandervorde.

Don't have a stroustrup book here but look for "Zero Initialization".

In the former book they say:

template <typename T>
void foo()
{
     T x = T(); // cannot use T x = 0; for classes!
}

in the std code they probably want to compare _U != 0
-->
_U != E(0)


modern compilers translate T() to 0 if T is int, e.g.

hope that helps,
arnulf

looking forward to ANTLR3 in C++ :)

Am 09.11.2010 14:26, schrieb David Wigg:
> I updated the CPP_parser from Antlr1 to Antlr2. Ramin Zaghi has done a
> great job to convert that to run under Antlr3. However, we do still
> have a problem with understanding one particular piece of code.
>
> This is in an include file, iosfwd (1998). shown below. The line in question is
> for (_N = 0; !eq(*_U, _E(0)); ++_U)
> and I would like to know what _E(0) is in order to able to parse it
> properly. It must be something that returns a value (like a pointer?).
> It looks to me like very early C code for a typecast but a later
> version (2007) of iosfwd has something similar, "_Elem()", in that
> position so I suppose it can't be a typecast.
> Is it a function? It can't be a class because a class doesn't return a value.
>
> I hope some C++ expert can help. If so, it would be a great help if
> he/she could quote chapter and verse from Stroustrup's book (Third
> Edition, or indeed any other) as well since that would help to get our
> C++ grammar file correct..
>
> Thanks.
>
> David Wigg
>
> template<class _E>
>          struct char_traits {
>          typedef _E char_type;
>          typedef _E int_type;
>          typedef streampos pos_type;
>          typedef streamoff off_type;
>          typedef mbstate_t state_type;
>          static void __cdecl assign(_E&  _X, const _E&  _Y)
>                  {_X = _Y; }
>          static bool __cdecl eq(const _E&  _X, const _E&  _Y)
>                  {return (_X == _Y); }
>          static bool __cdecl lt(const _E&  _X, const _E&  _Y)
>                  {return (_X<  _Y); }
>          static int __cdecl compare(const _E *_U, const _E *_V, size_t _N)
>                  {for (size_t _I = 0; _I<  _N; ++_I, ++_U, ++_V)
>                          if (!eq(*_U, *_V))
>                                  return (lt(*_U, *_V) ? -1 : +1);
>                  return (0); }
>          static size_t __cdecl length(const _E *_U)
>                  {size_t _N;
>                  for (_N = 0; !eq(*_U, _E(0)); ++_U)  // I would like
> to know what _E(0) is to parse it
>                          ++_N;
>                  return (_N); }
>          static _E * __cdecl copy(_E *_U, const _E *_V, size_t _N)
>                  {_E *_S = _U;
>                  for (; 0<  _N; --_N, ++_U, ++_V)
>                          assign(*_U, *_V);
>                  return (_S); }
>          static const _E * __cdecl find(const _E *_U, size_t _N,
>                  const _E&  _C)
>                  {for (; 0<  _N; --_N, ++_U)
>                          if (eq(*_U, _C))
>                                  return (_U);
>                  return (0); }
>          static _E * __cdecl move(_E *_U, const _E *_V, size_t _N)
>                  {_E *_Ans = _U;
>                  if (_V<  _U&&  _U<  _V + _N)
>                          for (_U += _N, _V += _N; 0<  _N; --_N)
>                                  assign(*--_U, *--_V);
>                  else
>                          for (; 0<  _N; --_N, ++_U, ++_V)
>                                  assign(*_U, *_V);
>                  return (_Ans); }
>          static _E * __cdecl assign(_E *_U, size_t _N, const _E&  _C)
>                  {_E *_Ans = _U;
>                  for (; 0<  _N; --_N, ++_U)
>                          assign(*_U, _C);
>                  return (_Ans); }
>          static _E __cdecl to_char_type(const int_type&  _C)
>                  {return ((_E)_C); }
>          static int_type __cdecl to_int_type(const _E&  _C)
>                  {return ((int_type)_C); }
>          static bool __cdecl eq_int_type(const int_type&  _X,
>                  const int_type&  _Y)
>                  {return (_X == _Y); }
>          static int_type __cdecl eof()
>                  {return ((-1)); }
>          static int_type __cdecl not_eof(const int_type&  _C)
>                  {return (_C != eof() ? _C : !eof()); }
>          };
>
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