In this bug report I have asked for better error messages: http://d.puremagic.com/issues/show_bug.cgi?id=6696

But beside the error message, do you know why an immutable ref can't be given to a function with a const ref argument? foo() can't change the contents of the array a any way, so what's wrong in this code?


void foo(const ref int[5] a) {}
void main() {
    immutable int[5] arr;
    foo(arr); // Error?
}

Bye,
bearophile

bearophile , dans le message (digitalmars.D.learn:29609), a écrit :
> what's wrong in this code?
> 
> 
> void foo(const ref int[5] a) {}
> void main() {
>     immutable int[5] arr;
>     foo(arr); // Error?
> }

I don't think it is wrong. Did you try changind the order of const and ref, or adding parenthesis ?

Christophe:

> I don't think it is wrong. Did you try changind the order of const and ref, or adding parenthesis ?

I am trying now, and it seems the situation doesn't change.

Bye,
bearophile

bearophile Wrote:

> In this bug report I have asked for better error messages: http://d.puremagic.com/issues/show_bug.cgi?id=6696
> 
> But beside the error message, do you know why an immutable ref can't be given to a function with a const ref argument? foo() can't change the contents of the array a any way, so what's wrong in this code?
> 
> 
> void foo(const ref int[5] a) {}
> void main() {
>     immutable int[5] arr;
>     foo(arr); // Error?
> }

Looks like a bug. Seems like the compiler treats casted argument as an rvalue and you can't pass rvalue byref, can you?

On Tue, 20 Sep 2011 07:33:20 -0400, bearophile <bearophileHUGS@lycos.com> wrote:

> In this bug report I have asked for better error messages:
> http://d.puremagic.com/issues/show_bug.cgi?id=6696
>
> But beside the error message, do you know why an immutable ref can't be given to a function with a const ref argument? foo() can't change the contents of the array a any way, so what's wrong in this code?
>
>
> void foo(const ref int[5] a) {}
> void main() {
>     immutable int[5] arr;
>     foo(arr); // Error?
> }

The complaint from the compiler is that cast(const(int[5u])) arr is not an lvalue.

So apparently, the compiler wants to *copy* arr (via cast), then ref that.

This smells like a bug, but could potentially be intended behavior.

There is a workaround, but it's UGLY and dangerous:

foo(*(cast(const(int[5])*)&arr));

I think the rules for implicit casting need to take into account whether it can use this kind of rewrite.

I'd file a bug, but note that it could possibly be an enhancement.

BTW, there are some funky things that happen with an immutable storage class.  For example:

void foo(ref const(int) a) {}
void bar(immutable int a) { foo(a); }
void main()
{
   immutable int a = 5;
   foo(a); // error, see below
   bar(a); // ok
}

The error for the foo(a) line is:
Error: constant 5 is not an lvalue

So the compiler is apparently replacing a with an enum.  But if it's a parameter, it gets storage on the stack.  That doesn't seem right.  I thought immutable data was supposed to live where you declare it, and that's why you use enum to force it to be a compile-time constant.

The same trick does not work with a fixed-size array, so I think definitely the original issue is a bug (behavior should be consistent with int).

-Steve

On Tue, 20 Sep 2011 07:33:20 -0400, bearophile <bearophileHUGS@lycos.com> wrote:

> In this bug report I have asked for better error messages:
> http://d.puremagic.com/issues/show_bug.cgi?id=6696
>
> But beside the error message, do you know why an immutable ref can't be given to a function with a const ref argument? foo() can't change the contents of the array a any way, so what's wrong in this code?
>
>
> void foo(const ref int[5] a) {}
> void main() {
>     immutable int[5] arr;
>     foo(arr); // Error?
> }


BTW, when posting questions like this, it is *immensely* helpful to give exact error messages, so I don't have to try it out to see what you are talking about.  I spent a while writing a response until I saw the error message (which changed completely how I viewed the issue), and erased it before writing my eventual reply.

-Steve

"Steven Schveighoffer" <schveiguy@yahoo.com> wrote in message news:op.v13w8td2eav7ka@localhost.localdomain...
> On Tue, 20 Sep 2011 07:33:20 -0400, bearophile <bearophileHUGS@lycos.com> wrote:
>
>> void foo(const ref int[5] a) {}
>> void main() {
>>     immutable int[5] arr;
>>     foo(arr); // Error?
>> }
>
> The complaint from the compiler is that cast(const(int[5u])) arr is not an lvalue.

It's a bug, the compiler shouldn't be inserting a cast when the value implicitly converts.  It's also a bug when the compiler tries to optimise away the variable to a literal when passing by reference, I've got a patch for this I haven't written up yet.

Steven Schveighoffer:

> BTW, when posting questions like this, it is *immensely* helpful to give exact error messages,

I have forgotten to do that by mistake. I am sorry.

Bye,
bearophile