Re: const-immutable array argument?

2011-09-21 Thread bearophile 
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

Re: const-immutable array argument?

2011-09-20 Thread Christophe 
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 ?

Re: const-immutable array argument?

2011-09-20 Thread bearophile 
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

Re: const-immutable array argument?

2011-09-20 Thread Kagamin 
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?

Re: const-immutable array argument?

2011-09-20 Thread Steven Schveighoffer 
On Tue, 20 Sep 2011 07:33:20 -0400, bearophile bearophileh...@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

Re: const-immutable array argument?

2011-09-20 Thread Steven Schveighoffer 
On Tue, 20 Sep 2011 07:33:20 -0400, bearophile bearophileh...@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

Re: const-immutable array argument?

2011-09-20 Thread Daniel Murphy 
Steven Schveighoffer schvei...@yahoo.com wrote in message 
news:op.v13w8td2eav7ka@localhost.localdomain...
 On Tue, 20 Sep 2011 07:33:20 -0400, bearophile bearophileh...@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.

Re: const-immutable array argument?

2011-09-20 Thread bearophile 
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