structs are now lvalues - what is with "auto ref"?

[Jonathan M Davis]

On Saturday, December 29, 2012 19:43:36 Minas Mina wrote:
> So when it will be fixed I will be able to write:
> 
> void foo(auto ref Vector3 v);
> 
> and it will pass copies or references depending on the situation?

For templated functions, it currently generates either

void foo(ref Vector3 v);

or

void foo(Vector3 v);

depending on whether you pass it an lvalue or an rvalue. The situtation with 
non-templated functions has not been sorted out yet, but what will probably 
happen once it has been is that if you declare

void foo(auto ref Vector3 v);

then underneath the hood, you'll only get

void foo(ref Vector3 v);

but when it's passed an rvalue, it will get automatically assigned to a local 
variable first and then that local variable will leave scope after the function 
call. So,

foo(bar());

will turn into something like

auto _temp = bar();
foo(_temp);

In either case, the idea is that if you declare a parameter to be auto ref, it 
will accept both rvalues and lvalues and do so without making unnecessary 
copies, whereas ref will still specifically require an lvalue. So, you use ref 
when you want to mutate the original, and you use auto ref when you don't care 
whether it gets mutated or not but don't want a copy to be made if it doesn't 
have to be (and you use auto ref const if you want to avoid the copy but 
guarantee that it doesn't get mutated).

- Jonathan M Davis