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Am 27.10.2012 21:12, schrieb Juha Nieminen:
> SG <> wrote:
>>> If you have a non-templated rvalue reference parameter,
>>> you can't give it an lvalue. You'll get a compile error.
>>
>> What is an "rvalue reference parameter"?
>
> Something like this:
>
> void foo(int&& i);
>
> If you try to give an lvalue to that, it gives an error.

Right. The thing is, if you write

template<class T> void blah(T&& x) {...}

x is not necessarily an rvalue reference parameter. It looks like one.
And it may be one depending on what T is. But is also may be an lvalue
reference. The reference collapsing rule "&+&&=&" makes x an lvalue
reference in case T is an lvalue reference type, otherwise x is an
rvalue reference. That's reference collapsing.

In addition to reference collapsing we have a special template argument
deduction rule which automatically picks the "right T" to make
initializing a parameter of type T&& possible. That is, if the argument
was an lvalue, the corresponding template type parameter is deduced to
be an lvalue reference type, otherwise it is a non-reference type. Why?
Because these rules are the rules they came up with to enable "perfect
forwarding".

> I must admit, I do not know the new standard enough to understand what
> exactly is going on here, and why it makes a difference (and why you need
> a typedef):
>
>> typedef int& foo;
>>
>> void bar(foo&& x);

The syntax does not allow "int& && x". I guess that is because there
would be no point in allowing it since you could just as well write
"int& x".

If you havn't already seen one of Scott Meyers' latest talks about
rvalue references, I encourage you to take a look.

HTH,
SG

Le 27/10/2012 21:12, Juha Nieminen a écrit :
> SG <> wrote:
>
> I must admit, I do not know the new standard enough to understand what
> exactly is going on here

To modify this unsatisfactory situation, you just have to read the
references I gave in my preceding message :

Thomas Becker paper :

http://thbecker.net/articles/rvalue_...ection_01.html

and Scott Meyers paper in Overload 111 october 2012 :

http://accu.org/index.php/journals/c78/

Ptyxs

Le 27/10/2012 21:12, Juha Nieminen a écrit :
> SG <> wrote:
>
> I must admit, I do not know the new standard enough to understand what
> exactly is going on here

To modify this unsatisfactory situation, you just have to read the
references I gave in my preceding message :

Thomas Becker paper :

http://thbecker.net/articles/rvalue_...ection_01.html

and Scott Meyers paper in Overload 111 october 2012 :

http://accu.org/index.php/journals/c78/

Ptyxs

On Friday, October 26, 2012 5:23:15 AM UTC-5, Juha Nieminen wrote:
> template<typename T> void foo(T&&, T&&);

Let's stop repeating the materials all over the internet. The
following is exactly what you want:

template<typename T1, typename T2> void foo(T1&&, T2&&,
typename std::enable_if<std::is_same<
typename std::remove_reference<T1>::type,
typename std::remove_reference<T2>::type>::value>::type* = 0)
{}