Re: Casting of pointer to member functions

From:

"John Carson" <jcarson_n_o_sp_am_@netspace.net.au>

Newsgroups:

microsoft.public.vc.language

Date:

Fri, 15 Sep 2006 23:30:48 +1000

Message-ID:

<#N4tusM2GHA.3908@TK2MSFTNGP05.phx.gbl>

"David Olsson" <vonolsson@gmail.com> wrote in message
news:1158322911.471685.111790@b28g2000cwb.googlegroups.com

I am writing an application in which I use pointer to member functions
to achieve a certain functionality. At one point I need to be able to
cast between different types of member function. In a very simplified
version, this is what I want to do:

class Base {
public:
  typedef void (Base::*MethodPrototype)();

  Base(MethodPrototype method): m_method(method) {}

  void call() {
     (this->*m_method)();
  }
private:
  MethodPrototype m_method;
};

class Derived: virtual public Base {
public:
  Derived():
Base(reinterpret_cast<MethodPrototype>(&Derived::myMethod)) {}

  int myMethod() {
     return(1);
  }
};

The constructor of the Derived class failes to compile as visual c++
reports that the reinterpret_cast is invalid. This does however only
occur when the inheritance is virtual, if it isn't virtual, the
casting works fine. Furthermore, the above code compiles with GCC
3.4.2. Is it Visual C++ or GCC that isn't quite conformant.

Comeau compiles it. Section 5.2.10/6 of the C++ standard says this about
reinterpret_cast:

"A pointer to a function can be explicitly converted to a pointer to a
function of a different type. The effect of calling a function through a
pointer to a function type (8.3.5) that is not the same as the type used in
the definition of the function is undefined. Except that converting an
rvalue of type "pointer to T1" to the type "pointer to T2" (where T1 and T2
are function types) and back to its original type yields the original
pointer value, the result of such a pointer conversion is unspecified.
[Note: see also 4.10 for more details of pointer conversions. ]"

My reading of this is that your Derived constructor should compile. However,
your call() function involves undefined behaviour when m_method is
initialised by the Derived constructor.

--
John Carson