ref const array error

[jdrewsen]

On Wednesday, 18 January 2012 at 20:13:04 UTC, Timon Gehr wrote:
> On 01/18/2012 08:59 PM, jdrewsen wrote:
>> On Wednesday, 18 January 2012 at 19:43:52 UTC, Timon Gehr 
>> wrote:
>>> On 01/18/2012 08:31 PM, jdrewsen wrote:
>>>> Recently the encoding.safeDecode stopped working for some of 
>>>> my existing
>>>> code. This example outlines the issue:
>>>>
>>>> import std.encoding;
>>>>
>>>> void main(string[] args) {
>>>> auto e = EncodingScheme.create("utf-8");
>>>> auto a = new byte[100];
>>>> e.safeDecode(a);
>>>> }
>>>>
>>>> Results in:
>>>>
>>>> Error: function std.encoding.EncodingScheme.safeDecode (ref
>>>> const(ubyte)[] s) const is not callable using argument types 
>>>> (byte[])
>>>>
>>>> Isn't this an error in the compiler?
>>>>
>>>> /Jonas
>>>>
>>>
>>> No, this is a bugfix. The operation is unsound:
>>>
>>> immutable(ubyte)[] foo(ref const(ubyte)[] s){
>>> auto r = new immutable(ubyte)[1];
>>> s = r;
>>> return r;
>>> }
>>>
>>> void main() {
>>> ubyte[] x;
>>> immutable(ubyte)[] y = foo(x);
>>> static assert(is(typeof(y[0])==immutable));
>>> auto oldy0 = y[0];
>>> x[0]=oldy0+1;
>>> assert(oldy0 == y[0]); // fail
>>> }
>>>
>>> The functionality is not going away; You will be able to use 
>>> inout for
>>> the same purpose once my enhancement request gets implemented:
>>> http://d.puremagic.com/issues/show_bug.cgi?id=7105
>>
>> Wouldn't a nicer solution be to let the compiler ensure that
>> an immutable array cannot escape through a ref const array 
>> parameter?
>>
>> /Jonas
>>
>
> That would not suffice.
>
> ubyte[] foo(ref const(ubyte)[] s){
>   auto r = new ubyte[1];
>   s = r;
>   return r;
> }
>
> void main() {
>   immutable(ubyte)[] x;
>   ubyte[] y = foo(x);
>   static assert(is(typeof(x[0])==immutable));
>   auto oldx0 = x[0];
>   y[0]=oldx0+1;
>   assert(oldx0 == x[0]); // fail
> }

In the example foo is actually using the ref s parameter as an 
out parameter. The compiler could catch that you're doing this 
and show an error.

This would force you to let foo look like:

ubyte[] foo(out const(ubyte)[] s);

Wouldn't that fix it?