[cxx-abi-dev] Transfer modes for parameters and return values

[David Vandevoorde]

On Jun 3, 2009, at 5:23 PM, Lawrence Crowl wrote:

> On 6/3/09, David Vandevoorde <daveed at edg.com> wrote:
>> Hello again,
>>
>> In 3.1.1 and 3.1.4, the ABI specifies that "by value" class type  
>> parameters
>> and class type return values are passed via the address of a  
>> reference if
>> the class type has
>>        (a) a nontrivial destructor, or
>>        (b) a nontrivial copy constructor.
>>
>> Should we now also add to that:
>>        (c) a (nontrivial) move constructor
>> ?
>>
>> (There is currently no notion of "trivial move constructor", but I  
>> think
>> there are suggestions to introduce that in the future.)
>
> I'm not sure I can predict the binary consequences of such a notion
> until it is actually defined.  So, I would rather wait until then.


It doesn't really have to be defined formally: A constructor for class  
X whose first parameter has type
	X cv-quals&&
and whose other parameters have a default argument or are the ellipsis  
parameter.  Such a constructor can be preferred over the trivial copy  
constructor for "rvalue copying" (i.e., moving) purposes.

	Daveed