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

[Richard Smith]

On Tue, May 21, 2013 at 10:00 AM, Gábor Kozár <kozargabor at gmail.com> wrote:

> I think it makes sense. For example, if you delete the copy ctor, the
> compiler can tell you - when you want to copy - that it's not possible,
> because the the copy ctor was deleted.
>
> In addition, and more importantly, it allows this to behave as expected:
>
> class Base {};
> class Derived : public Base {};
>
> struct Foo
> {
>     Foo(Base* o) { ... }
>     Foo(Derived* o) = delete;
> };
>
> Foo(new Derived); // error: that overload is deleted
>
> Now if the = delete would just "remove" that overload, then the last line
> would compile as a call to Foo(Base*), with an implicit derived-to-base
> cast.
>

I agree, but that's not the case we're talking about.


> Gabor
>
>
> 2013/5/16 David Blaikie <dblaikie at gmail.com>
>
>>
>>
>>
>> On Wed, May 15, 2013 at 9:03 PM, Richard Smith <richard at metafoo.co.uk>wrote:
>>
>>> 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)."
>>>
>>
>> OK, that's weird. How is that different to having it not provided at all?
>> (& it seems really unfortunate that deleted definitions do anything other
>> than cause compilation errors - affecting overload resolution, etc, adds
>> some substantial wrinkles to the model)
>>
>>
>>>
>>> Clang doesn't implement this correctly in all cases yet, though.
>>>
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>>>
>>
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>
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