Re: std::pair copy and move constructor

[François Dumont <frs dot dumont at gmail dot com>]

On 03/05/2013 11:03 PM, Daniel Krügler wrote:
> 2013/3/5 Jonathan Wakely <jwakely.gcc@gmail.com>:
> > On 5 March 2013 20:36, François Dumont wrote:
> > >      I finally wonder if the compiler couldn't help here. If the copy
> > > assignment is deleted why wouldn't the compiler try to use the template
> > > assignment operator ?
> > If an operation is deleted that means the class author (implicitly or
> > explicitly) said "this operation is not allowed"
> >
> > If the compiler finds a way to work around the author's intention by
> > choosing a different function then what's the point in having deleted
> > functions?
> My understanding of Francois' suggestion would allow to declare the
> special members as defaulted, and to provide the template versions
> only as the fall-back. In this case the effect would be that
> std::pair<int, double> would use the defaulted copy assignment
> operator (cao), so the default is fine. In case of std::pair<int&,
> double>, the defaulted coa would be deleted and the compiler would
> still find the (constrained) template assignment operator (tao),
> having the effect of the current user-defined one. In case of
> std::pair<const int, double> the cao is again deleted, the compiler
> tries the tao (which can be designed to reject this cases as well),
> and we have a fully constrained type. This sounds reasonable to me and
> it emulates the effect of a built-in rule for a valid copy-assignment
> operator for references to the best possible way. I also understand
> him to restrict this rule to the special member functions. This
> approach looks like an extension of CWG 1402 to me (which introduces a
> similar rule for move special members). I wouldn't like to have this
> rule for other functions and it would still not be possible to
> constrain a destructor, but that is life ;-)
>
> - Daniel
    That's indeed what I had in mind. And yes, limited to special 
methods, I can't even imagine how it could be generalized to others.

    Note that in the case of std::pair<cont int, double> the template 
assignment operator would fail to compile too so it would be 
transparent. The only difference would be that 
std::is_copy_assignable<std::pair<const int, double>> would give the 
correct result. Add std::is_copy_assignable<std::pair<int&, double>> too 
which is of course std::false_type. So it would be very nice.

    Interesting the point about the destructor.

François