References

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On Friday, 20 September 2013 at 09:36:18 UTC, andrea9940 wrote:
> Running this code I would expect to get "ref" three times, but 
> ...
>
> -------
> import std.stdio;
> struct A {
> 	int[128] data;
> 	ref A opAdd(const ref A a) {
> 		A cp = this;
> 		cp.data[] += a.data[];
> 		return cp;
> 	}
> }
>
> void fun(A a) { writeln("copy"); }
> void fun(const ref A a) { writeln("ref"); }
>
> void main() {
> 	A a, b;
> 	fun(a + b); //prints "copy"
> 	fun(A());   //prints "copy"
> 	fun(a);     //prints "copy"
This prints 'ref' if you change func(A a) to func(const A a) the 
match of const ref isn't prefered over A a because const need an 
implicit conversion.
> }
> -------
>
> After some tests I concluded that is not possible to pass the 
> result of a+b as a reference (which is what interests me most) 
> and this creates an evident performance problem.
> Is there any solution that I missed ?

a + b is an rvalue and will moved to func(A a). This is even 
faster than ref. ;)
A() is also moved.
If you have two functions, one with const A a and one with ref 
const A a, the latter accepts lvalues which are passed by ref and 
the first accepts rvalues which are moved. So no performance 
problem. ;)
If you're function is a template you can use 'auto ref' which 
automatically generate two versions of your function: one with 
and one without ref.