Re: [boost] [move] one more dumb question

Subject: Re: [boost] [move] one more dumb question
From: Jeffrey Hellrung (jhellrung_at_[hidden])
Date: 2010-02-25 16:20:56

DE wrote:
> how do i make a function return an object by value and other object
> to, say, move assign from that returned object within current C++ std?
> i.e.
> 
>   my_type foo() {/*...*/}
>   //...
>   my_type a;
>   a = foo();  //move assignment is intended here
> 
> --
> Pavel

Fortunately, you don't have to worry about the return type of foo; just 
return by value, as you normally would.  Return Value Optimization (RVO) 
(or whatever mechanism you want to attribute this to) takes care of 
eliding the copy between the "return" in foo and the call site.  So now 
you just need to worry about which operator= the assignment will call.

Currently, I believe, if you use the move-enabling macros in Boost.Move, 
then the move assignment operator of my_type will be chosen for the 
assignment operation, rather than the copy assignment operator.  The 
"trick" that Boost.Move employs is to declare the copy assignment 
operators as operator=(T&) and operator=(const rv<T>&); and the move 
assignment operator as operator=(rv<T>&).  The end result is that 
rvalues of type T bind to the move assignment operator, while all 
lvalues will bind to one of the copy assignment operators.  Thus, the 
above assignment, "a = foo()", will move assign.

There has been some discussion, however, that this mechanism has 
undesirable side effects, one of which is the "poisoning" of the 
auto-generated copy assignment operator for all classes that enclose a 
class (inheritance or member object) that use a move-enabling macro. 
The auto-generated copy assignment operator will take its parameter by 
reference to non-const, which is Bad.

The alternative is, for move-enabled classes, to write a single 
assignment operator accepting its parameter by value.  This is safe, 
specifically addressing the above "poisoning" problem, if non-optimal 
for classes with member objects for which copying and moving is identical.

I've actually experimented with different alternatives to the 
move-enabling macros (as well as how to implement forwarding), and at 
some point soon will present my thoughts to Ion on what I think could be 
done.

- Jeff