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From: Andrea Torsello (torsello_at_[hidden])
Date: 2004-04-16 11:09:19

David Abrahams wrote:

> "Andrea Torsello" <torsello_at_[hidden]> writes:
>> Hi, I have been investigating move semantics and think that I came up
>> with a solution that is strongly influenced by both David Abrahams' move
>> approach and Andrei Alexandrescu's mojo, but solves some of the problems
>> I have encountered with them.
>> The basic idea is to use the explicit keyword to force the compiler to
>> use
>> the X(X const &) constructor only in two cases.
>> 1) When explicitly copying a const object i.e.
>> X const x;
>> X y(x); //use explicit copy constructor
> Not when passing a non-const lvalue?
> X x;
> X y(x); // implicitly uses a move ctor?? That's highly undesirable!

Actually, as described in a folowup, the approach works only with copy
initialization, not with direct initialization. Direct initialization
always uses the X(X const&) copy constructor. Not a major drawback in my
use-cases, but your milage may vary.

When using the copy initialization
X x;
X y=x;

the X(X&) constructoor is used.

>> 2) When passing by const ref
>> void f(X const &);
>> X makeX();
>> void foo()
>> {
>> f(makeX()); //use explicit copy constructor
>> }
>> In all other cases the other constructor take precedence.
> Which "other constructor"?

Should have been other constructorS.
i.e. X(X&) for non-const lvalues, X(X::constant) for constants and
X(X::temporary) for non-const rvalues.

>> Note that the template trick David Abrahams uses in his move
>> approach would lead to an error on a conforming compiler when
>> confronted with case #2 (unless more recent versions corrected
>> this).
> I'm not sure what you mean, but AFAICT case #2 is handled just fine by
> conforming compilers. Did you read
> It's a question of interpretation of a particular clause of the
> standard.

Not sure about the standard's wording, but Comeau in strict mode fails to
compile the test you provided on this list some time ago.

Furthermore in the text of that test you added a comment after test 10, and
I quote
"// This one fails in Comeau's strict mode due to 12.2/1. GCC 3.3.1 passes
it erroneously."
Finally the need for X(X const &) to pass a temporary by const ref was the
reason that forced Alexandrescu to add the special return type to mojo.

>> I chose to keep the temporary and constant subtypes from mojo because
>> they allow me to discriminate between const values, non-const rvalues and
>> non-const rvalues.
> const and non-const rvalues are pretty much interchangeable, since the
> compiler can turn the former into the latter. If they weren't
> interchangeable, I don't see any reason you'd want to treat a const
> rvalue differently from a const lvalue.
>> The lack of this capability was the major problem I had with David
>> Abrahams' move.
> Why?

Passing by value is fine if you always move from temporaries and copy from
non-temporaries: you pass by value and move from the passed object. If,
like in the case of my vector-like container, you don't want to change the
backing store unless you have to, the pass-by-value-and-move approach is
not feasible

> My solution allows discrimination between temporaries and
> non-temporaries.

It certainly does on copying, but does it allow to pick different overloaded
functions when passing a temporary and a non-temporary? The code snipped
you sent on the quoted message doesn't appear to allow it, but as I said, I
am not sure about newer versions.

>> My solution fullfills almost all the requirements I had:
>> 1) passing by value moves non-const rvalues and copies everithing else.
>> 2) its is transparent to the user of the class.
>> Alexandrescu's Mojo fails this because of the special return type,
>> while
>> Abrahams' move fails it because of the pass-by-const-ref problem
>> described above.
> Please describe the problem in more detail.
>> 3) it allows to discriminate between constants, non-const lvalues, and
>> non-const rvalues.
>> Alexandrescu's Mojo allows it, while Abrahams' move does not.
> Why is that an advantage?

I Hope I explained the points clearly enough, but please contact me if you
have any doubt criticism or enquiry. I value all the feedback you can

Thanks a lot
Andrea Torsello

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