Re: c++/9278: dependent type in conversion operator bug

[David Abrahams <dave@boost-consulting.com>]

The following reply was made to PR c++/9278; it has been noted by GNATS.

From: David Abrahams <dave@boost-consulting.com>
To: Wolfgang Bangerth <bangerth@ticam.utexas.edu>
Cc: "Joseph S. Myers" <jsm28@cam.ac.uk>,  <gcc-bugs@gcc.gnu.org>,
	  <gcc-gnats@gcc.gnu.org>
Subject: Re: c++/9278: dependent type in conversion operator bug
Date: Mon, 13 Jan 2003 21:25:30 -0500

 Wolfgang Bangerth <bangerth@ticam.utexas.edu> writes:
 
 >> > Indeed a very obscure feature. (Although, I must admit that, I desperately 
 >> > needed something like that not too long ago and had to invent various 
 >> > template metaprogramming hoops to work around...)
 >> 
 >> I needed exactly that yesterday, which is why it came up ;-)
 >> Using SFINAE to rule out certain conversions was my game, but it
 >> didn't fly :(
 >
 > SFINAE???
 
 "Substitution Failure Is Not An Error"
 
 basically you can take an overload out of the overload set by causing
 an invalid type computation in one of the arguments or return type:
 
 template <bool = true, class T>
 struct enable_if { typedef T type; };
 template <class T>
 struct enable_if<false,T> {};
 
 // Define addition for all enums, but only for enums
 template <class T>
 typename enable_if<boost::is_enum<T>::value, T>::type
 operator+(T x, T y) { return T(x+y); }
 
 > In my case, I wanted to make things more uniform in Functor classes: when 
 > you call a function, it either returns a value, or void. So this does not
 > go together
 >   RETTYPE ret_val = function(args)
 > if RETTYPE==void. What an annoying difference, and how convenient would it 
 > be if there were "void variables", i.e. objects to which you can assign 
 > the result of a void expression :-)
 
 Oh.  But you are allowed to write "return function(args);" even if
 function returns void.
 
 > Some template trickery and specilization can get one around this, however.
 
 Been there, done that.
 
 -- 
                        David Abrahams
    dave@boost-consulting.com * http://www.boost-consulting.com
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