Re: std::pair copy and move constructor

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

On 02/16/2013 01:21 PM, Daniel Krügler wrote:
> 2013/2/16 François Dumont<frs.dumont@gmail.com>:
> > On 02/15/2013 10:00 PM, Daniel Krügler wrote:
> > > 2013/2/15 François Dumont<frs.dumont@gmail.com>:
> > > > Hi
> > > >
> > > >       I had a problem with the result of
> > > > std::is_copy_assignable<std::pair<const int, int>>::type which used to be
> > > > true_type. So here is a patch to fix that.
> > > This patch would break with the requirements of the library. In
> > > particular it would prevent that std::pair's of references are
> > > copy-assignable and that exactly is the purpose for the user-provided
> > > copy-assignment operators of pair (and tuple).
> > I knew there must have been something.
> >
> > Is there a test showing this problem ?
> I would expect that the following code would no longer be accepted:
>
> #include <utility>
>
> int main() {
>    int i{};
>    int j{};
>    std::pair<int&, int&> pii1(i, j);
>    const std::pair<int&, int&> pii2(i, j);
>    pii1 = pii2;
> }
>
> because with a defaulted copy-assignment operator, std::pair<int&,
> int&> would have a deleted copy-assignment operator.
    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 ? Is there something in the Standard saying that 
only the copy assignment operator can be used ? If so, couldn't the 
Standard be amended to allow this behavior ?

     It sounds like a natural behavior when the copy assignment 
operator is default implemented and when this default implementation 
can't be generated that the compiler consider other challengers. In the 
case of std::pair<int&, int&> it even makes a lot of sens to use the 
template assignment operator as the currently manually written copy 
assignment operator behaves like a simple assignment operator as soon as 
a reference type is involved.

François