Hi,

Just started in D a week back after years of C/C++ head bashing and have to say language is fantastic.

I have a small misunderstanding regarding const...well ok, a fundamental misunderstanding by the looks of it:)

class A
{

ulong[] values;
...
const ulong[] getValue() const {
    return this.values;
}

}

But this fails to compile with the following message:
Error: cannot implicitly convert expression (this.values) of type const(ulong[]) to ulong[]

I just do not see it in the code. The function is const, so this.values is const, and I am just returning that as a const ulong[]. Am I not???


I don't get it? Any help explaining why this does not compile would be appreciated!

Thanks,
Stewart

Sorry all, I just need to read the docs more carefully and do more homework! Especially the section "Immutable Member Functions" under Const and Immutable.


...couldn't be more obvious :-)

Cheers,
Stewart

On Tuesday, 29 January 2013 at 05:36:31 UTC, estew wrote:
> Hi,
>
> Just started in D a week back after years of C/C++ head bashing and have to say language is fantastic.
>
> I have a small misunderstanding regarding const...well ok, a fundamental misunderstanding by the looks of it:)
>
> class A
> {
>
> ulong[] values;
> ...
> const ulong[] getValue() const {
>     return this.values;
> }
>
> }
>
> But this fails to compile with the following message:
> Error: cannot implicitly convert expression (this.values) of type const(ulong[]) to ulong[]
>
> I just do not see it in the code. The function is const, so this.values is const, and I am just returning that as a const ulong[]. Am I not???
>
>
> I don't get it? Any help explaining why this does not compile would be appreciated!
>

Hi,

This is a very common syntax trap where too many people fall. When you do const ulong[] getValue() const both const are qualifying this, and none the return value. So the error.

The correct declaration is :
const(ulong)[] getValue() const

or

const(ulong[]) getValue() const

I wish this syntax will be fixed, but I have no hope. We just will see the problem popping again and again I guess.

deadalnix:

> I wish this syntax will be fixed, but I have no hope. We just will see the problem popping again and again I guess.

I asked for that fix, but Walter closed down that request in Bugzilla. I still think he was wrong.

I now open a smaller request that can't fix the problem fully:
http://d.puremagic.com/issues/show_bug.cgi?id=9422

Bye,
bearophile

Could you link to the issue Walter closed down from issue 9422?

Kenji Hara


2013/1/29 bearophile <bearophileHUGS@lycos.com>

> deadalnix:
>
>
>  I wish this syntax will be fixed, but I have no hope. We just will see
>> the problem popping again and again I guess.
>>
>
> I asked for that fix, but Walter closed down that request in Bugzilla. I still think he was wrong.
>
> I now open a smaller request that can't fix the problem fully: http://d.puremagic.com/issues/**show_bug.cgi?id=9422<http://d.puremagic.com/issues/show_bug.cgi?id=9422>
>
> Bye,
> bearophile
>

On Tuesday, 29 January 2013 at 09:10:28 UTC, bearophile wrote:
> deadalnix:
>
>> I wish this syntax will be fixed, but I have no hope. We just will see the problem popping again and again I guess.
>
> I asked for that fix, but Walter closed down that request in Bugzilla. I still think he was wrong.
>
> I now open a smaller request that can't fix the problem fully:
> http://d.puremagic.com/issues/show_bug.cgi?id=9422
>

Well I think the error message you propose isn't possible as it would cause many problems in generic code.

const applied to the rturn type should be the way to go. It is basically what everybody except at first when looking at the code. But it is probably too late to change that.

On 01/29/2013 10:38 AM, deadalnix wrote:
> On Tuesday, 29 January 2013 at 09:10:28 UTC, bearophile wrote:
>> deadalnix:
>>
>>> I wish this syntax will be fixed, but I have no hope. We just will
>>> see the problem popping again and again I guess.
>>
>> I asked for that fix, but Walter closed down that request in Bugzilla.
>> I still think he was wrong.
>>
>> I now open a smaller request that can't fix the problem fully:
>> http://d.puremagic.com/issues/show_bug.cgi?id=9422
>>
>
> Well I think the error message you propose isn't possible as it would
> cause many problems in generic code.
>

How?

> const applied to the rturn type should be the way to go. It is basically
> what everybody except at first when looking at the code. But it is
> probably too late to change that.

The current behaviour emerges because the parser treats

const void foo(){}

exactly like

const{
    void foo(){}
}

kenji hara:

> Could you link to the issue Walter closed down from issue 9422?

I have taken part of that discussion thread, but the bug report was created by Simen Kjaeraas:

http://d.puremagic.com/issues/show_bug.cgi?id=4070

(Maybe in Bugzilla there is another similar closed issue, because I vaguely remember Jonathan commenting on it.)

Bye,
bearophile

deadalnix:

> Well I think the error message you propose isn't possible as it would cause many problems in generic code.

Please, add one or two of such troubled cases in the issue thread:

http://d.puremagic.com/issues/show_bug.cgi?id=9422

Bye,
bearophile

On Tuesday, 29 January 2013 at 09:50:37 UTC, bearophile wrote:
> deadalnix:
>
>> Well I think the error message you propose isn't possible as it would cause many problems in generic code.
>
> Please, add one or two of such troubled cases in the issue thread:
>
> http://d.puremagic.com/issues/show_bug.cgi?id=9422
>
> Bye,
> bearophile

For what it's worth, I think 9422 is better than 4070. You rarely see const on the return type without also putting it on the function too, so this should catch *most* accidental usage.

It also still allows the syntax for those that enjoy qualifying everything on the previous line:
//----
pure @property @safe const
int foo()
{
    ......
}
//----