Re: [boost] Fatal problem for ZUTO/MOCO/MOJO or bug in standard?

From: David Abrahams (dave_at_[hidden])
Date: 2002-10-16 10:22:21

"Kevin S. Van Horn" <Kevin.VanHorn_at_[hidden]> writes:

> A Mojo-enabled class X has ctors
> 
>   X(X&);
>   X(mojo::constant<X>);
>   X(mojo::temporary<X>);
> 
> but does NOT have a ctor
> 
>   X(X const &);
> 
> Now, section 12.8, paragraph 1 of the Standard states
> 
> "A class object can be copied in two ways, by initialization (12.1, 8.5),
> including for function argument passing (5.2.2) and for function value 
> return (6.6.3), and by assignment (5.17).  Conceptually, THESE TWO 
> OPERATIONS ARE IMPLEMENTED BY A COPY CONSTRUCTOR (12.1) and COPY 
> ASSIGNMENT OPERATOR (13.5.3)."
> 
> (Emphasis added.)  Section 12.1, paragraph 10 defines a copy ctor:
> 
> "A copy constructor for a class X is a constructor with a first parameter 
> of type X& or of type const X&."
> 
> This implies that to return an rvalue of type X from a function, or to 
> pass an rvalue as a function argument that is to be of type X, a 
> constructor X(const X&) must exist and be accessible, since a copy ctor us 
> used to return or pass the arguments, this ctor must take X& or X const &,
> and the X& form cannot be used with temporaries.
> But mojo-enabled classes don't have X const & ctors.  Oops.
> 
> But neither does auto_ptr<T>!  So either there's a flaw in my reasoning, 
> or there's a flaw in the Standard.

Well, we shouldn't be discussing MOJO in this forum, since Andrei
asked us not to. The issue is the same for auto_ptr, as you say. For
this answer, I'd say you should ask Bill Gibbons (bill_at_[hidden]).

-- 
           David Abrahams * Boost Consulting
dave_at_[hidden] * http://www.boost-consulting.com