Const ref and rvalues again...

[luka8088]

On 13.11.2012 15:07, martin wrote:
> On Tuesday, 13 November 2012 at 08:34:19 UTC, luka8088 wrote:
>>> Your proposal isn't really related to this thread's topic
>>
>> Um, "Const ref and rvalues again", I suggest it to be the default
>> behavior, how is this not related ?
>
> The topic here is binding rvalues to (const) ref parameters. You, on the
> other hand, are suggesting to flip the constness of by-value parameters
> (int => val/mutable int, const int => int), which affects both rvalues
> and lvalues (no difference between them) and only by-value parameters.
>
>> Yes, you understood correctly:
>> void f (const ref int x, int y, ref int z); =>
>> void f (int x, val int y, ref int z);
>>
>> The point here is to make "We need a way for a function to declare
>> that it doesn't want it's argument to be copied, but it also doesn't
>> care whether the argument is an rvalue or an lvalue. " a default
>> behavior.
>
> So now tell me why argument x wouldn't be copied. It's passed by value,
> so of course it is copied (lvalues)/moved (rvalues) just as it is now.
> The only difference is that the parameter won't be modified by f().
>
> I guess what you furthermore implicate is that you'd expect the compiler
> to automatically pass appropriate arguments to such parameters by
> reference to avoid copying (for large structs or structs with
> non-trivial copy constructors). Such a (handy!) optimization is sadly
> not possible due to aliasing issues, e.g.:
>
> int foo(ref int dst, const int src)
> {
> dst = 2*src;
> return src;
> }
> // "optimized" foo():
> int bar(ref int dst, const ref int src)
> {
> dst = 2*src;
> return src;
> }
>
> int i = 1;
> assert(foo(i, i) == 1 && i == 2); // okay
> i = 1;
> assert(bar(i, i) == 2 && i == 2); // wtf?!
> // the const src parameter is actually modified since the
> // original argument i is also used as mutable dst parameter!
>
>> Would it ? How many functions actually change their non ref/out
>> arguments ? Can you point out any existing public code that would be
>> broken ?
>
> I don't want to look for examples in Phobos etc. as it should be trivial
> to imagine cases such as:
>
> void bla(float x)
> {
> // restrict x to safe range [0,1]
> x = max(0, min(1, x));
> }

I see, you are correct, if it is not copied then it can be changed 
before function finished through some other references hence it must be 
copied.