Re: [google] Check if the nonnull attribute is applied to 'this' (issue4446070)

[Basile Starynkevitch <basile at starynkevitch dot net>]

On Fri, 29 Apr 2011 11:08:24 -0400 (EDT)
dnovillo@google.com (Diego Novillo) wrote:

> This patch from Le-Chun Wu adds support to check whether a nonnull
> attribute is applied to 'this' pointer for non-static methods.

This bring me to a question. Does the C++ standard imply that this is
never a null pointer? (Last time I looked into a C++ standard, I was
not able to give a firm answer).

In other words, with 
  class Myclass {
     int x;
   public:
     Myclass(int z=0): x(z) {};
     int get(void) const { if (this) return x; else return 0; };
     ~Myclass() { };
  };
can a C++ standard conforming compiler optimize the get () member
function to 
  inline int Myclass::get(void) { return x; };
(I tend to believe that such an optimization is incorrect, but I am not
sure). can a C++ program call
   Myclass *t = null_ptr;
   return t->get();
(I might believe this is not conforming to standards)

I feel that testing that this is not null is occasionnally useful. For
instance, if I coded a trivial interpreter (e.g. of a Lua or Python or
Lisp-like language) I would be tempted, if that language has a nil
value, to implement values as pointers to objects and to represent nil
as a C++ null_ptr. So I might even want to code something like

// my abstract top superclass for values
class Value {
  virtual ~Value() {};
  Value () {};
  virtual void do_print(void) const = 0;
public:
  void print(void) const { if (this) this->do_print(); };
};

class IntegerValue : public Value {
  int x;
public:
  IntegerValue (int z) : Value(), x(z) {}:
  virtual void do_print (void) { cout << x; };
};

But I am not sure that such code is standard C++. However, GCC seems to
compile it as I expect! At least I still hope that future versions of
GCC would accept it (perhaps in some permissive mode, or with warnings)

Regards.

PS: By standard C++, I mean some recent or future C++ ISO standard (eg C++1x or C++0x).

-- 
Basile STARYNKEVITCH         http://starynkevitch.net/Basile/
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