Re: decltype(*this) - C++0x

From:
Stuart Golodetz <blah@blah.com>
Newsgroups:
comp.lang.c++
Date:
Wed, 29 Sep 2010 12:28:46 +0100
Message-ID:
<i7v7t6$pj$1@speranza.aioe.org>
Fernando wrote:

Hi people,

I want to write something like this example ( C++0x )

class comparable
{
public:
   bool is_equal( decltype(*this) other ) // should be X&
   {
       return true; // no matter
   }
};

I get the following errors

MSVC10 -> Error 1 error C2355: 'this' : can only be
referenced inside non-static member functions
MinGW (GCC 4.6) -> invalid use of 'this' at top level

In the n1478.pdf paper (
http://std.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1478.pdf ) are examples
that use decltype with "this" keyword, but not as an argument.

class X {
   void foo() {
      decltype(this) // X*
      decltype(*this) // X&
      ...
  }
  void bar() const {
     decltype(this) // const X*
     decltype(*this) // const X&
     ...
  }
};

Sorry for the comparisons, but in the Eiffel language you can write
something like this ...

    is_equal (other: like Current) : BOOLEAN

Current is the same that C++ this.
other argument will be the same type as the class where the method is
written.

I wonder whether this alternative decltype (*this) is left out for some
specific reason or if you know any other way to achieve the same result.

Thanks,
Fernando Pelliccioni.


D'oh -- just realised this was cross-posted to the moderated group
(possibly not a good idea, since the posts of anyone who accidentally
replies to both won't come through in comp.lang.c++ until they pass
moderation). For what it's worth, the reply I just sent said:

===

One way of achieving the same result here might be CRTP:

#include <iostream>

class Comparable
{
public:
     virtual ~Comparable() {}
     virtual bool is_equal(const Comparable& rhs) const = 0;
};

template <typename T>
class TypedComparable : public Comparable
{
public:
     bool is_equal(const Comparable& rhs) const
     {
         const T *p = dynamic_cast<const T*>(&rhs);
         if(p) return typed_is_equal(*p);
         else return false;
     }

     virtual bool typed_is_equal(const T& rhs) const = 0;
};

class X : public TypedComparable<X>
{
private:
     int m_i;
public:
     X(int i)
     : m_i(i)
     {}
public:
     bool typed_is_equal(const X& rhs) const
     {
         return m_i == rhs.m_i;
     }
};

class Y : public TypedComparable<Y>
{
public:
     bool typed_is_equal(const Y& rhs) const
     {
         return true;
     }
};

int main()
{
     X x1(23), x2(23), x3(9);
     Y y;
     std::cout << x1.is_equal(x2) << ' ' << x1.is_equal(x3) << ' ' <<
x1.is_equal(y) << ' ' << y.is_equal(y) << '\n';
     return 0;
}

Not sure if that helps at all?

Cheers,
Stu

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