alignof(type, field); sizeof(type, field); typeof(type, field): getting type information on nested field

[Yann Droneaud <ydroneaud at opteya dot com>]

Hi,

I'm sometime in need to "probe" the size, the type, (and less often the
alignment) of a field inside a structure.

In such case I have to write "ugly" thing like

  struct A
  {
    struct
    {
      type_t t;
    } B;
  };

  typeof(((struct A *)NULL)->B.t) V;

It would have been some much pleasing to have 2 parameters version of
sizeof(), typeof(), and alignof(), which would look like a lot like
offsetof() usage:

  typeof(struct A, B.t) V;

  if (sizeof(struct A, B.t) != sizeof(long)) ... ;

  if (alignof(struct A, B.t) < alignof(long)) ... ;

As sizeof is an operator, this is not straightforward to implement.

Currently sizeof (0, variable); is a valid construct, giving the size
of  "variable".

Hence, turning sizeof() to a variadic function-like thing would break
existing code.

Maybe it would possible to add such function-like operator as _Sizeof()
? And have a header #define'ing sizeof(...) as _Sizeof() for code
wanting the new behavior ? 

Is this possible ? Is such feature was already considered ?

Regards.

-- 
Yann Droneaud
OPTEYA