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Re: Templated Casting operators

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Narinder wrote:

[...]
> Consider the following code:
> 
> -------------------------------------------------------------
> 
> #include<iostream>
> using namespace std;
> 
> double pi =3.142;
> 
> struct klass
> {
> 
> template<class T>
> operator T&()
> {
> cout << "I am NON-const version\n";
> return pi;
> }
> 
> template<class T>
> operator const T&()const
> {
> cout << "I am CONST version\n";
> return pi;
> }
> 
> 
> };
> 
> template<class T>
> T& show(T &t)
> {
> cout << "NON-CONST show\n";
> return t;
> }
> 
> template<class T>
> const T& show(const T &t)
> {
> cout << "CONST show\n";
> return t;
> }
> 
> int main()
> {
> 
> klass k;
> show(k);
> show(klass());
> 
> double x1 = k;
> double x2 = klass();
> 
> }
> 
> -----------------------------------------------------
> 
> Output from msvc :
> NON-CONST show
> CONST show
> I am CONST version
> I am CONST version
> 
> Output from gcc
> NON-CONST show
> CONST show
> I am NON-CONST version
> I am NON-CONST version
> 
> To me the msvc appears 'correct' or at least self-consistent. Or
> perhaps I am missing something.
> 
> Any thoughts ?

I think, gcc gets it right and lets you observe a language quirk: In this 
version, the two conversion operators differ by constness. Thus, overload 
resolution should be based on constness of the klass object being converted:

  klass k; // declares a non-const klass object k
  show(k); // should choose the T& version
  show( klass() ); // should choose the T const & version because of 8.5.3/5
  double x1 = k; // should choose the non-const member as k is non-const
  double x2 = klass(); // should choose the non-const member as the 
                       // temporary klass() is non-const

The last issue is an inconsistency in the language: though klass() is non-
const, the temporary cannot be used to initialize a non-const reference.


Best,

Kai-Uwe Bux

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Thread

Templated Casting operators Narinder <narinder.claire@gmail.com> - 2011-07-07 16:34 -0700
  Re: Templated Casting operators Alexander Bartolich <alexander.bartolich@gmx.at> - 2011-07-08 00:58 +0000
    Re: Templated Casting operators Narinder <narinder.claire@gmail.com> - 2011-07-07 22:36 -0700
      Re: Templated Casting operators Kai-Uwe Bux <jkherciueh@gmx.net> - 2011-07-08 09:42 +0200
        Re: Templated Casting operators Narinder <narinder.claire@gmail.com> - 2011-07-08 01:16 -0700
          Re: Templated Casting operators Ian Collins <ian-news@hotmail.com> - 2011-07-08 20:23 +1200
        Re: Templated Casting operators Narinder <narinder.claire@gmail.com> - 2011-07-08 01:37 -0700
          Re: Templated Casting operators Narinder <narinder.claire@gmail.com> - 2011-07-08 06:28 -0700
        Re: Templated Casting operators Narinder <narinder.claire@gmail.com> - 2011-07-08 01:40 -0700
          Re: Templated Casting operators Kai-Uwe Bux <jkherciueh@gmx.net> - 2011-07-09 00:06 +0200
            Re: Templated Casting operators Narinder <narinder.claire@gmail.com> - 2011-07-09 03:16 -0700
              Re: Templated Casting operators Kai-Uwe Bux <jkherciueh@gmx.net> - 2011-07-09 13:34 +0200
                Re: Templated Casting operators Narinder <narinder.claire@gmail.com> - 2011-07-09 07:18 -0700
                Re: Templated Casting operators Kai-Uwe Bux <jkherciueh@gmx.net> - 2011-07-10 01:15 +0200
                Re: Templated Casting operators Narinder <narinder.claire@gmail.com> - 2011-07-09 21:39 -0700
                Re: Templated Casting operators Narinder <narinder.claire@gmail.com> - 2011-07-09 22:01 -0700
                Re: Templated Casting operators Kai-Uwe Bux <jkherciueh@gmx.net> - 2011-07-10 13:43 +0200
                Re: Templated Casting operators Narinder <narinder.claire@gmail.com> - 2011-07-10 05:55 -0700
                Re: Templated Casting operators Narinder <narinder.claire@gmail.com> - 2011-07-10 00:19 -0700
                Re: Templated Casting operators Kai-Uwe Bux <jkherciueh@gmx.net> - 2011-07-10 12:03 +0200

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