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From "Eric Lemings" <Eric.Lemi...@roguewave.com>
Subject RE: error on tuple copy ctor
Date Wed, 02 Jul 2008 22:38:46 GMT
 

> -----Original Message-----
> From: Martin Sebor [mailto:msebor@gmail.com] On Behalf Of Martin Sebor
> Sent: Wednesday, July 02, 2008 3:55 PM
> To: dev@stdcxx.apache.org
> Subject: Re: error on tuple copy ctor
> 
> Eric Lemings wrote:
> >  
> > 
> >> -----Original Message-----
> >> From: Martin Sebor [mailto:msebor@gmail.com] On Behalf Of 
> Martin Sebor
> >> Sent: Wednesday, July 02, 2008 3:18 PM
> >> To: dev@stdcxx.apache.org
> >> Subject: Re: error on tuple copy ctor
> >>
> > ...
> >>> Should we add the ctor even if the standard does not 
> >> (currently) specify it?
> >>
> >> I don't think it's needed or desirable. In the test case I
> >> posted, we want to call the const T& overload.
> > 
> > That's the only workaround I can think of.  You have another one in
> > mind?
> 
> A workaround for what? This is a valid definition of
> a CopyConstructible and MoveConstructible class (like tuple):
> 
>      struct S {
>          S (const S&);
>          S (S&&);
>      };
> 
> I don't see why S would need another copy ctor with the signature
> of S(S&). I realize tuple is quite a bit more complicated than S,
> too complicated for me to understand why the ctor might be
> necessary if, if fact, it really is. Could you show in a small
> isolated example the problem that this ctor works around?

It would probably bind as expected if not for the template ctors in
tuple.  Apparently the compiler will prefer binding to a template ctor
any chance it gets because the constructed type and argument types are
"better", if not exact, matches in a template ctor.  Not so for trivial
copy ctors.

For binding to a `S (const S&)' ctor given an object value of S, there
is an implicit const conversion involved.

Here's a small test case to illustrate:

	#include <iostream>
	using namespace std;

	struct S {
	    S () {}

	    S (const S&) { cout << "trivial copy ctor called" << endl; }

	    template <class T>
	    S (T&&) { cout << "templated move ctor called" << endl; }
	};

	int main () {
	    S s1;
	    S s2 (s1); // trivial copy or templated move?
	    return 0;
	}

Brad.

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