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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 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