[cfe-dev] A question about when a defaulted move assignment operator is deleted

[Richard Smith]

This is not a forum for learning about C++; stackoverflow.com would be a
better place to ask such questions.

On Wed, May 15, 2013 at 8:42 PM, ZhangXiongpang <zhangxiongpang at gmail.com>wrote:

> Platform: linux, x86_84, clang++3.3 (trunk 178517), g++4.7.2
>
> I'm learning C++11 standard, and often write some code to test clang++.
> But sometimes I'm not sure whether my understanding is right when clang++
> does not work as my expecting.
>
> 12.8/p23 in N3290:
> --------------------------------------------------------
> A defaulted copy/move assignment operator for class X is defined as deleted
> if X has:
>   ...
>   -- for the move assignment operator, a non-static data member or direct
> base class with a type that does
>      not have a move assignment operator and is not trivially copyable, or
> any direct or indirect virtual
>      base class.
> --------------------------------------------------------
> Does it partially mean that the defaulted move assignment operator for
> class
> X is defined as deleted if X has any direct or indirect virtual base class?
>

In the latest draft of the standard, that bullet has been removed. (Since
virtual bases can be assigned multiple times by defaulted assignment
operators, this means that the compiler might generate a broken move
assignment operator for classes which inherit from the same virtual base
through multiple inheritance paths. I argued against this, but the
committee seemed to prefer the simpler rule.)

We also have this (which doesn't help in your example, but is relevant in
general):

"A defaulted move assignment operator that is defined as deleted is ignored
by overload resolution (13.3, 13.4)."

Clang doesn't implement this correctly in all cases yet, though.
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