ref const(T) the same as C++'s const T&?

[Peter Alexander]

On 15/02/11 7:55 PM, Jonathan M Davis wrote:
> On Tuesday, February 15, 2011 11:48:25 Peter Alexander wrote:
>> Do D const references work the same as C++'s?
>>
>> i.e.
>> - Can they bind to rvalues?
>> - Do they extend the life of rvalues?
>>
>> If they do, are there any differences from C++?
>>
>> If they don't, how do I pass large structs into a function efficiently?
>
> The only bind to lvalues. A ref is a reference to a variable, so normal ref has
> to be passed a variable. In C++, it was decided that const&  was special and
> could take a temporary. In D, it's no different from ref except that it's const,
> so it can't be changed. There have been requests to fix this, but Andrei is
> adamant that it was a huge mistake in C++ to do it since it makes it so that you
> can't determine whether a const&  is really a rvalue or an lvalue. I don't know
> enough to know how much that really matters, but apparently it causes problems.
> The result is that D doesn't do that, which can be rather annoying when trying
> to pass temporaries to functions which take const ref, but that's the way it
> goes.
>
> If you want to pass a large struct to a function efficiently, and you know that
> function isn't going to need to alter the struct, make it const ref. That way,
> it's const, and it doesn't get copied. However, that means that you can't pass
> temporaries to it. However, if you need to pass a temporary, then you're going
> to have to actually assign it to a variable first.
>
> I _thought_ tht you could define a cost ref version and a non-const ref version
> and it would call the const ref version on lvalues and the non-const ref version
> on rvalues, but when I just tried it, it seems to call the non-const ref version
> in both cases. I don't know if that's a bug or not. But if you just have a const
> ref version, and you create a variable where you'd normally use a temporary,
> you'll be fine.
>
> - Jonathan M Davis

Surely this is a major usability issue? I have to create a new local 
variable just so that I can pass structs round efficiently (either that 
or change the struct to have COW or ref semantics, both of which are 
unacceptable).