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digitalmars.D.learn - const->immutable array argument?

reply bearophile <bearophileHUGS lycos.com> writes:
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
Sep 20 2011
next sibling parent reply travert phare.normalesup.org (Christophe) writes:
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 ?
Sep 20 2011
parent bearophile <bearophileHUGS lycos.com> writes:
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
Sep 20 2011
prev sibling next sibling parent Kagamin <spam here.lot> writes:
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?
Sep 20 2011
prev sibling next sibling parent reply "Steven Schveighoffer" <schveiguy yahoo.com> writes:
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
Sep 20 2011
parent "Daniel Murphy" <yebblies nospamgmail.com> writes:
"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.
Sep 20 2011
prev sibling next sibling parent reply "Steven Schveighoffer" <schveiguy yahoo.com> writes:
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
Sep 20 2011
parent bearophile <bearophileHUGS lycos.com> writes:
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
Sep 20 2011
prev sibling parent bearophile <bearophileHUGS lycos.com> writes:
Daniel Murphy:


 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. 


http://d.puremagic.com/issues/show_bug.cgi?id=6708

Thank you for all the answers.

Bye,
bearophile
Sep 21 2011