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November 27, 2012
auto ref and arrays
I don't understand why auto ref doesn't work with arrays.

void test1(T)(auto ref const T[] val) {}
void test2(T)(auto ref const T val) {}
void main() {
  int b;
  test2(b); // OK
  string a;
  test1(a); // Error: cast(const(char[]))a is not an lvalue
}

Since a is mutable itself, compiler uses ref storage class.
cast(const(char[]))a isn't an lvalue, so it's impossible to pass 
it by ref.

But cast(const int)b isn't an lvalue too. Why it's no errors in 
this case?
November 27, 2012
Re: auto ref and arrays
On 11/27/2012 02:09 PM, Jack Applegame wrote:
> I don't understand why auto ref doesn't work with arrays.
>
> void test1(T)(auto ref const T[] val) {}
> void test2(T)(auto ref const T val) {}
> void main() {
> int b;
> test2(b); // OK
> string a;
> test1(a); // Error: cast(const(char[]))a is not an lvalue
> }
>
> Since a is mutable itself, compiler uses ref storage class.
> cast(const(char[]))a isn't an lvalue, so it's impossible to pass it by ref.
>
> But cast(const int)b isn't an lvalue too. Why it's no errors in this case?

Sorry, I am not answering the question but is 'auto ref' is legal as a 
parameter? It is not documented.

On the other hand, 'auto ref' is a legal return type.

Ali
November 27, 2012
Re: auto ref and arrays
On Tuesday, November 27, 2012 14:44:51 Ali Çehreli wrote:
> On 11/27/2012 02:09 PM, Jack Applegame wrote:
> > I don't understand why auto ref doesn't work with arrays.
> > 
> > void test1(T)(auto ref const T[] val) {}
> > void test2(T)(auto ref const T val) {}
> > void main() {
> > int b;
> > test2(b); // OK
> > string a;
> > test1(a); // Error: cast(const(char[]))a is not an lvalue
> > }
> > 
> > Since a is mutable itself, compiler uses ref storage class.
> > cast(const(char[]))a isn't an lvalue, so it's impossible to pass it by
> > ref.
> > 
> > But cast(const int)b isn't an lvalue too. Why it's no errors in this case?
> 
> Sorry, I am not answering the question but is 'auto ref' is legal as a
> parameter? It is not documented.
> 
> On the other hand, 'auto ref' is a legal return type.

Yes, it's legal on function paramters - but only on templated functions. It 
was an attempt at solving the const& problem. It's supposed to take both 
lvalues and rvalues but not copy lvalues. I think that there's a decent chance 
that TDPL talks about it, but I'm not sure, though if it does, what it 
describes isn't quite what auto ref does at the moment, since the original 
idea for auto ref would have worked with non-templated functions, but Andrei 
and Walter miscommunicated on its intent, and it wasn't implemented that way 
(and would actually be problematic to implement that way).

As for the OP's question, I'd say that it looks like a bug. My guess is that a 
cast is being inserted as part of the auto ref implementation, and a cast does 
_not_ result in an lvalue, so that's why it would fail. I don't know why it 
thinks it needs a cast though.

- Jonathan M Davis
November 27, 2012
Re: auto ref and arrays
On 11/27/12 23:44, Ali Çehreli wrote:
> On 11/27/2012 02:09 PM, Jack Applegame wrote:
>> I don't understand why auto ref doesn't work with arrays.
>>
>> void test1(T)(auto ref const T[] val) {}
>> void test2(T)(auto ref const T val) {}
>> void main() {
>> int b;
>> test2(b); // OK
>> string a;
>> test1(a); // Error: cast(const(char[]))a is not an lvalue
>> }
>>
>> Since a is mutable itself, compiler uses ref storage class.
>> cast(const(char[]))a isn't an lvalue, so it's impossible to pass it by ref.
>>
>> But cast(const int)b isn't an lvalue too. Why it's no errors in this case?
> 
> Sorry, I am not answering the question but is 'auto ref' is legal as a parameter? It is not documented.

It's legal and documented: http://dlang.org/template.html

Whether the argument is mutable or not shouldn't matter -- compiler bug;
both examples should compile (limiting auto-ref to only modifiable lvalues
wouldn't make sense).

artur
November 28, 2012
Re: auto ref and arrays
http://d.puremagic.com/issues/show_bug.cgi?id=9090
November 28, 2012
Re: auto ref and arrays
On Tuesday, 27 November 2012 at 22:09:21 UTC, Jack Applegame 
wrote:
> I don't understand why auto ref doesn't work with arrays.
>
> void test1(T)(auto ref const T[] val) {}
> void test2(T)(auto ref const T val) {}
> void main() {
>   int b;
>   test2(b); // OK
>   string a;
>   test1(a); // Error: cast(const(char[]))a is not an lvalue
> }
>
> Since a is mutable itself, compiler uses ref storage class.
> cast(const(char[]))a isn't an lvalue, so it's impossible to 
> pass it by ref.
>
> But cast(const int)b isn't an lvalue too. Why it's no errors in 
> this case?

I thought with auto ref you dispense with the const since it 
figures it out.
The below seems to work.

Thanks,
Dan

import std.traits;
import std.stdio;

void test1(T)(auto ref T val) if(!isArray!T) {
  writeln("Nonarray ", typeid(typeof(val)));
}
void test1(T)(auto ref T val) if(isArray!T) {
  writeln("Array ", typeid(typeof(val)));
}

void main() {
  string a;
  const(string) b = "string that never changes";
  test1(a);
  test1(b);

  int[] x;
  test1(x);
  int y;
  test1(y);
}
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